In chemistry, the values ​​​​of the absolute masses of molecules are not used, but the value of the relative molecular mass is used. It shows how many times the mass of a molecule is greater than 1/12 of the mass of a carbon atom. This value is denoted by M r .

The relative molecular weight is equal to the sum of the relative atomic masses of its constituent atoms. Calculate the relative molecular weight of water.

You know that a water molecule contains two hydrogen atoms and one oxygen atom. Then its relative molecular mass will be equal to the sum of the products of the relative atomic mass of each chemical element and the number of its atoms in a water molecule:

Knowing the relative molecular weights of gaseous substances, one can compare their densities, i.e., calculate the relative density of one gas from another - D (A / B). The relative density of gas A for gas B is equal to the ratio of their relative molecular masses:

Calculate the relative density of carbon dioxide for hydrogen:

Now we calculate the relative density of carbon dioxide for hydrogen:

D(co.g./hydrogen.) = M r (co. g.) : M r (hydrogen.) = 44:2 = 22.

Thus, carbon dioxide is 22 times heavier than hydrogen.

As you know, Avogadro's law applies only to gaseous substances. But chemists need to have an idea about the number of molecules and in portions of liquid or solid substances. Therefore, to compare the number of molecules in substances, chemists introduced the value - molar mass .

Molar mass is denoted M, it is numerically equal to the relative molecular weight.

The ratio of the mass of a substance to its molar mass is called amount of matter .

The amount of a substance is denoted n. This is a quantitative characteristic of a portion of a substance, along with mass and volume. The amount of a substance is measured in moles.

The word "mole" comes from the word "molecule". The number of molecules in equal amounts of a substance is the same.

It has been experimentally established that 1 mol of a substance contains particles (for example, molecules). This number is called Avogadro's number. And if you add a unit of measurement to it - 1 / mol, then it will be a physical quantity - the Avogadro constant, which is denoted N A.

Molar mass is measured in g/mol. The physical meaning of the molar mass is that this mass is 1 mole of a substance.

According to Avogadro's law, 1 mole of any gas will occupy the same volume. The volume of one mole of gas is called the molar volume and is denoted by V n .

Under normal conditions (and this is 0 ° C and normal pressure - 1 atm. Or 760 mm Hg or 101.3 kPa), the molar volume is 22.4 l / mol.

Then the amount of gas substance at n.o. can be calculated as the ratio of gas volume to molar volume.

TASK 1. What amount of substance corresponds to 180 g of water?

TASK 2. Let us calculate the volume at n.o., which will be occupied by carbon dioxide in the amount of 6 mol.

Bibliography

1. Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. "Chemistry, Grade 8" / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. - M.: AST: Astrel, 2006. (p. 29-34)
2. Ushakova O.V. Chemistry workbook: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. Grade 8” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006. (p. 27-32)
3. Chemistry: 8th grade: textbook. for general institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005. (§§ 12, 13)
4. Chemistry: inorg. chemistry: textbook. for 8 cells. general institution / G.E. Rudzitis, F.G. Feldman. - M .: Education, JSC "Moscow textbooks", 2009. (§§ 10, 17)
5. Encyclopedia for children. Volume 17. Chemistry / Chapter. edited by V.A. Volodin, leading. scientific ed. I. Leenson. - M.: Avanta +, 2003.

1. A single collection of digital educational resources ().
2. Electronic version of the journal "Chemistry and Life" ().
3. Chemistry tests (online) ().

Homework

1.p.69 No. 3; p.73 Nos. 1, 2, 4 from the textbook "Chemistry: 8th grade" (P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M .: AST: Astrel, 2005).

2. №№ 65, 66, 71, 72 from the Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. "Chemistry, Grade 8" / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. - M.: AST: Astrel, 2006.

Instruction

If you carefully consider the table of Dmitry Ivanovich Mendeleev, you can see that it looks like a multi-apartment multi-storey building, in which there are "residents" - elements. Each of them has a surname () and chemical. Moreover, each of the elements lives in its own apartment, and therefore has. This information is presented in all cells of the table.

However, there is another figure, at first glance, completely incomprehensible. Moreover, it is indicated with several values ​​after the decimal point, which is done for greater accuracy. It is this number that you need to pay attention to, because this is the relative atomic mass. Moreover, this is a constant value that does not need to be memorized and can be found in the table. By the way, even at the exam according to D.I. Mendeleev is a reference material available for use, and each one is in an individual package - KIM.

The molecular mass, or rather the relative substance, denoted by the letters (Mr) is made up of the relative atomic masses (Ar) that form the molecule, the elements. Relative atomic mass is just that mysterious figure that stands in every cell of the table. For calculations, these values ​​must be rounded up to a whole number. The only exception is the chlorine atom, whose relative atomic mass is 35.5. This characteristic has no units of measurement.

Example 1. Find the molecular mass(KOH)
The potassium hydroxide molecule consists of one potassium atom (K), one oxygen atom (O), and one hydrogen atom (H). Therefore, we find:
Mr (KOH) \u003d Ar (K) + Ar (O) + Ar (H)


Hence: Mr (KOH) = 39 + 16 + 1 = 56

Example 2. Find the molecular mass sulfuric acid (H2SO4 ash-two-es-o-four)
The sulfuric acid molecule consists of two hydrogen atoms (H), one sulfur atom (S), and four oxygen atoms (O). Therefore, we find:
Mr(H2SO4) = 2Ar(H) + Ar(S) + 4Ar(O)
According to the table D.I. Mendeleev, we find the values ​​of the relative atomic masses of the elements:
Ar (K) = 39, Ar (O) = 16, Ar (H) = 1
Hence: Mr (H2SO4) = 2 x 2 + 32 + 4 x 16 = 98

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note

When calculating, multiplication or division is performed first, and only then addition or subtraction is performed.

Helpful advice

When determining the relative atomic mass, round off the values ​​that are in the D.I. table. Mendeleev to an integer

Sources:

• how to calculate molecular weight
• Molecular weight definition

To find the molecular mass, find the molar mass substances in grams per mole, since these quantities are numerically equal. Or search mass particles of a molecule in atomic mass units, add their values ​​​​and get the molecular mass. To find the molecular weight of a gas, you can use the Clapeyron-Mendeleev equation.

You will need

• For calculations, you will need the periodic table of Mendeleev, scales, thermometer, pressure gauge.

Instruction

Calculation using the periodic table. Determine the chemical formula of the test substance. In the periodic table, find the chemical elements that make up the molecule. In the corresponding cells, find their atomic mass. If the table represents a fractional number, round it up to a whole number. If the same element occurs several times in a molecule, multiply it mass for the number of entries. Add up all the atoms. The result is substances.

Calculation of molecular weight when converted from grams. If the mass of one molecule is given in grams, multiply it by Avogadro's constant, which is 6.022 10^(23) 1/mol. The result will be the substance in grams per mole. Its numerical value coincides with the molecular weight in atomic mass units.

Calculation of the molecular weight of an arbitrary gas. Take a cylinder of known volume measured in cubic meters, pump out air from it and weigh it on a balance. Then, pump gas into it, molecular mass which needs to be determined. find again mass balloon. The difference between the gas cylinder and the empty cylinder will be equal to the mass of gas, carry out in grams. Measure the pressure with a pressure gauge (at) and the temperature with a thermometer by converting it to . To do this, add the number 273 to the degrees Celsius obtained as a result of the measurement. To find the molar mass gas, his mass multiply by the temperature and the number 8.31 (universal gas constant). The result obtained is successively divided by the value of the gas pressure and its volume M = m 8.31 T / (P V). This indicator, expressed in grams per mole, is numerically the molecular weight of the gas, expressed in atomic mass units.

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Sources:

• molecular weight calculation

The relative molecular weight of a substance (or simply the molecular weight) is the ratio of the mass value of a given substance to 1/12 of the mass of one carbon atom (C). Find the relative molecular weight mass very easy.

You will need

• Periodic table and molecular weight table

Instruction

The relative of a substance is the sum of its atomic masses. In order to learn the atomic mass one way or another, just look at the periodic table. It can be found on the cover of any software, or purchased separately at a bookstore. For a pocket version, or an A4 sheet is quite suitable. Any modern chemistry is equipped with a full-scale wall periodic table.

Having learned the nuclear mass element, you can begin to calculate the molecular weight of the substance. This is easiest to show with an example:
It is required to calculate the molecular mass water (H2O). It can be seen from the molecular formula that a water molecule consists of two H atoms and one O atom. Therefore, the calculation of the molecular weight of water can be reduced to the action:
1.008*2 + 16 = 18.016

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note

Atomic mass as a concept appeared in 1803, thanks to the work of the then famous chemist John Dalton. In those days, the mass of any atom was compared with the mass of a hydrogen atom. This concept was further developed in the works of another chemist, Berzelius, in 1818, when he proposed using an oxygen atom instead of a hydrogen atom. Since 1961, chemists of all countries have taken as a unit of atomic mass the mass of 1/16 of an oxygen atom, or the mass of 1/12 of a carbon atom. The latter is just indicated in the periodic table of chemical elements.

Helpful advice

When using the periodic table in the form in which it is presented in most chemistry textbooks and other reference books, it must be understood that this table is a shortened version of the original periodic table. In its most complete version, a separate line is devoted to each chemical element.

The molecular weight of a substance refers to the total atomic mass of all the chemical elements that are part of that substance. To calculate the molecular mass substances, no special effort is required.

You will need

• Mendeleev table.

Instruction

Now you need to take a closer look at any of the elements in this table. Under the name of any of the elements indicated in the table there is a numerical value. It is it and the atomic mass of this element.

Now it is worth considering a few examples of calculating the molecular weight, based on the fact that the atomic masses are now known. For example, you can calculate the molecular weight of a substance such as water (H2O). A water molecule contains one oxygen atom (O) and two hydrogens (H). Then, having found the atomic masses of hydrogen and oxygen from the periodic table, we can begin to calculate the molecular mass: 2 * 1.0008 (after all, there are two hydrogens) + 15.999 = 18.0006 amu (atomic mass units).

Another . The next substance, molecular mass which can be calculated, let it be ordinary table salt (NaCl). As can be seen from the molecular formula, the salt molecule contains one Na atom and one chlorine Cl atom. In this case, it is considered as follows: 22.99 + 35.453 = 58.443 a.m.u.

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note

I would like to note that the atomic masses of the isotopes of various substances differ from the atomic masses in the periodic table. This is due to the fact that the number of neutrons in the nucleus of an atom and inside an isotope of the same substance is different, so the atomic masses also differ markedly. Therefore, isotopes of various elements are usually denoted by the letter of the given element, while adding its mass number in the upper left corner. An example of an isotope is deuterium ("heavy hydrogen"), the atomic mass of which is not one, like an ordinary atom, but two.

Molar is weight one mole of a substance, that is, such an amount of it that contains as many atoms as 12 grams of carbon. In another way, such a quantity is called the number (or constant) of Avogadro, in honor of the Italian scientist who first put forward the hypothesis. According to it, equal volumes of ideal gases (at the same temperatures and pressures) must contain the same number of molecules.

It must be firmly remembered that one mole of any substance is approximately 6.022 * 1023 molecules (either atoms or ions) of this substance. Therefore, any amount of any substance can be represented by elementary calculations in the form of a certain number of moles. And why was the mole introduced at all? To facilitate calculations. After all, the number of elementary (molecules, atoms, ions) even in the smallest sample of a substance is simply colossal! Agree, it is much more convenient to express the amount of substances in moles than in huge zeros with endless rows! Molar weight substance is determined by adding the molar masses of all the elements included in it, taking into account the indices. For example, you need to determine the molar mass of anhydrous sodium sulfate. First of all, write down its chemical formula: Na2SO4. Do the calculations: 23*2 + 32 + 16*4 = 142 grams/mol. This will be the molar weight this salt. And if you need to determine the molar mass of a simple substance? The rule is exactly the same. For example, molar weight oxygen O2 \u003d 16 * 2 \u003d 32 grams / mol, molar weight N2 \u003d 14 * 2 \u003d 28 grams / mol, etc. It is even easier to determine the molar mass, the molecule of which consists of one atom. For example, molar weight sodium is 23 / mol, silver - 108 grams / mol, etc. Of course, rounded values ​​are used here to simplify calculations. If greater accuracy, it is necessary for the same sodium to consider its relative atomic mass equal not to 23, but to 22.98. It must also be remembered that the value of the molar mass of a substance depends on its quantitative and qualitative composition. Therefore, different substances with the same number of moles have different molar masses.

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Tip 6: How to Determine Relative Molecular Weight

The relative molecular weight of a substance is a value showing how many times the mass of one molecule of a given substance is greater than 1/12 of the mass of the carbon isotope. In another way, it can be called simply molecular weight. How can you find the relative molecular mass?

You will need

• Mendeleev table.

Instruction

All you need for this is the periodic table and the elementary ability to make calculations. After all, the relative molecular mass is the sum of the atomic masses of the elements that make up the one you are interested in. Of course, taking into account the indices of each element. The atomic mass of each element is listed in the Periodic Table along with other important information, and with very high accuracy. For these purposes, rounded values ​​\u200b\u200bare quite suitable.

Now take the periodic table and determine the atomic masses of each element included in its composition. There are three such elements: , sulfur, . Atomic mass (H) \u003d 1, atomic mass of sulfur (S) \u003d 32, atomic mass of oxygen (O) \u003d 16. Given the indices, sum up: 2 + 32 + 64 \u003d 98. This is the relative molecular weight of sulfuric acid. Please note that this is an approximate, rounded result. If, for some reason, accuracy is required, then it will be necessary to take into account that the atomic mass of sulfur is not exactly 32, but 32.06, hydrogen is not exactly 1, but 1.008, etc.

note

If the Periodic Table is not at hand, find out the relative molecular weight of a particular substance using reference books on chemistry.

Helpful advice

The mass of a substance in grams, which is numerically equal to its relative molecular weight, is called a mole.

The relative molecular weight of a substance shows how many times a molecule of a given substance is heavier than 1/12 of an atom of pure carbon. It can be found if its chemical formula is known, using Mendeleev's periodic table of elements. Otherwise, use other methods to find the molecular weight, given that it is numerically equal to the molar mass of the substance, expressed in grams per mole.

You will need

• - periodic table of chemical elements;
• - hermetic container;
• - scales;
• - manometer;
• - thermometer.

Instruction

If a substance is known, determine its molecular weight using Mendeleev's periodic table of chemical elements. To do this, identify the elements that are in the formula of the substance. Then, find their relative atomic masses, which are recorded in the table. If the atomic mass in the table is a fractional number, round it up to the nearest whole number. If it contains several atoms of the given element, multiply the mass of one atom by their number. Add up the resulting atomic masses and get the relative molecular mass of the substance.

For example, to find the molecular weight of sulfuric H2SO4, find the relative atomic masses of the elements that are included in the formula, respectively, of sulfur and oxygen Ar(H)=1, Ar(S)=32, Ar(O)=16. Given that there are 2 atoms of hydrogen in a molecule, and 4 atoms of oxygen, calculate the molecular weight of the substance Mr(H2SO4)=2 1+32+4∙16=98 atomic mass units.

In the event that the amount of the substance in moles ν and the mass of the substance m, expressed in grams, are known, determine its molar mass; for this, divide the mass by the amount of the substance M=m/ν. It will be numerically equal to its relative molecular weight.

If you know the number of molecules of a substance N, the known mass m, find its molar mass. It will be equal to the molecular weight by finding the ratio of the mass in grams to the number of molecules of the substance in this mass, and multiply the result by the Avogadro constant NA = 6.022 ^ 23 1 / mol (M = m ∙ N / NA).

To find the molecular weight of an unknown gas, find its mass in a known sealed volume. To do this, pump the gas out of it by creating a vacuum there. Weigh. Then pump the gas back in and find its mass again. The difference between the masses of the empty and filled cylinder will be equal to the mass of the gas. Measure the pressure inside the cylinder with a pressure gauge in Pascals, and in Kelvins. To do this, measure the ambient temperature, it will be equal to the inside of the cylinder in degrees Celsius, to convert it to Kelvin, add 273 to the resulting value.

Determine the molar mass of a gas by finding the product of the temperature T, the mass of the gas m, and the universal gas constant R (8.31). Divide the resulting number by the values ​​​​of pressure P and volume V, measured in m³ (M \u003d m 8.31 T / (P V)). This number will correspond to the molecular weight of the gas under study.

Hydrogen is the first element of the periodic table and the most abundant in the universe, since it is from it that stars are mainly composed. It is part of the vital substance for biological life - water. Hydrogen, like any other chemical element, has specific characteristics, including its molar mass.

Instruction

Remember molar mass? This is the mass of one mole, that is, such an amount of it, in which there are approximately 6.022 * 10 ^ 23 elementary particles of matter (atoms, molecules, ions). This number is called "Avogadro's number", and is named after the famous scientist Amedeo Avogadro. The molar mass of a substance numerically coincides with its molecular mass, but has a different dimension: not atomic mass units (amu), but gram / mol. Knowing this, determine the molar mass hydrogen as easy as pie.

What has a molecule hydrogen? It is diatomic, with the formula H2. Immediately: a molecule is considered, consisting of two atoms of the lightest and most common hydrogen isotope, protium, and not of the heavier

Atoms and molecules are the smallest particles of matter, therefore, as a unit of measurement, you can choose the mass of one of the atoms and express the masses of other atoms in relation to the selected one. So what is molar mass, and what is its dimension?

What is molar mass?

The founder of the theory of atomic masses was the scientist Dalton, who compiled a table of atomic masses and took the mass of a hydrogen atom as a unit.

Molar mass is the mass of one mole of a substance. A mole, in turn, is the amount of a substance that contains a certain amount of the smallest particles that participate in chemical processes. The number of molecules in one mole is called Avogadro's number. This value is constant and does not change.

Rice. 1. Avogadro's number formula.

Thus, the molar mass of a substance is the mass of one mole, in which there are 6.02 * 10^23 elementary particles.

Avogadro's number got its name in honor of the Italian scientist Amedeo Avagadro, who proved that the number of molecules in equal volumes of gases is always the same.

Molar mass in the International SI system is measured in kg / mol, although this value is usually expressed in grams / mol. This value is denoted by the English letter M, and the molar mass formula is as follows:

where m is the mass of the substance and v is the amount of the substance.

Rice. 2. Calculation of the molar mass.

How to find the molar mass of a substance?

The table of D. I. Mendeleev will help to calculate the molar mass of a substance. Take any substance, for example, sulfuric acid. Its formula is as follows: H 2 SO 4. Now let's turn to the table and see what is the atomic mass of each of the elements that make up the acid. Sulfuric acid consists of three elements - hydrogen, sulfur, oxygen. The atomic mass of these elements, respectively, is 1, 32, 16.

It turns out that the total molecular weight is 98 atomic mass units (1 * 2 + 32 + 16 * 4). Thus, we found out that one mole of sulfuric acid weighs 98 grams.

The molar mass of a substance is numerically equal to the relative molecular mass if the structural units of the substance are molecules. The molar mass of a substance can also be equal to the relative atomic mass if the structural units of the substance are atoms.

Until 1961, the oxygen atom was taken as an atomic mass unit, but not the whole atom, but its 1/16 part. At the same time, the chemical and physical units of mass were not the same. The chemical one was 0.03% more than the physical one.

At present, a unified measurement system has been adopted in physics and chemistry. As a standard e.a.m. 1/12 of the mass of the carbon atom is chosen.

Rice. 3. Formula of the unit of atomic mass of carbon.

The molar mass of any gas or vapor is very easy to measure. It is enough to use control. The same volume of a gaseous substance is equal in quantity to another at the same temperature. A known way to measure the volume of steam is to determine the amount of displaced air. This process is carried out using a side outlet leading to the measuring device.

The concept of molar mass is very important in chemistry. Its calculation is necessary for the creation of polymer complexes and many other reactions. In pharmaceuticals, the concentration of a given substance in a substance is determined using molar mass. Also, the molar mass is important in the provision of biochemical studies (the exchange process in the element).

Nowadays, thanks to the development of science, the molecular weights of almost all blood components, including hemoglobin, are known.

Instruction

The unit of molecular weight is 1/12 of the mass of an atom, which is conventionally taken as 12. The molecular weight is the total relative atomic mass of all the atoms in the molecule, and it is very easy to calculate.

And there is the easiest option if you know the substance. Take the periodic table, look at the molecular weight of each element included in. For example, for hydrogen it is 1, - 16. And to find the molecular weight of the whole substance (let's take for example water, which consists of two hydrogen molecules and one), simply add the masses of all the elements included in it. For water: M(H2O) = 2M(H)+M(O) = 2 1+16 = 18 a. eat.

Helpful advice

As you can see, finding the molecular weight can be very simple. The main thing is not to confuse it with the molar mass of a substance - they are numerically equal to each other, but have different units of measurement and physical meaning.

Sources:

• Determine the molecular formula of a hydrocarbon if

Related videos

Sources:

• Experience as a teacher

In order to determine mass atom, find the molar mass of a monatomic substance using the periodic table. Then divide this mass by Avogadro's number (6.022 10^(23)). This will be the mass of the atom, in the units in which the molar mass was measured. The mass of an atom of a gas is found in terms of its volume, which is easy to measure.

You will need

• To determine the mass of an atom of a substance, take the periodic table, tape measure or ruler, pressure gauge, thermometer.

Instruction

Determining the mass of an atom of a solid body or To determine the mass of an atom of a substance, determine it (what it consists of). In the periodic table, find the cell that describes the corresponding element. Find the mass of one mole of this substance in grams per mole that is in this cell (this number corresponds to the mass of the atom in atomic mass units). Divide the molar mass of the substance by 6.022 10^(23) (Avogadro's number), the result is the given substance in grams. The mass of an atom can also be determined in another way. To do this, multiply the atomic mass of a substance in atomic mass units taken in the periodic table by the number 1.66 10^(-24). Get the mass of one atom in grams.

Determining the mass of an atom of a gas In the event that there is an unknown gas in the vessel, determine its mass in grams by weighing the empty vessel and the vessel with gas, and find the difference between their masses. After that, measure the volume of the vessel using a ruler or tape measure, followed by calculations or other methods. Express the result in . Use a manometer to measure the pressure of the gas inside the vessel, and measure its temperature with a thermometer. If the thermometer scale is calibrated in Celsius, determine the temperature value in Kelvin. To do this, add the number 273 to the temperature value on the thermometer scale.

Determining the molar mass of a substance from the mass of a molecule If you know the mass of one molecule in grams, multiply it by the Avogadro number 6.022 10^(23), which is equal to the number of molecules in one mole of the substance. The result will be the substance in grams per mole. Having found it in the periodic table, if necessary, determine the substance itself, if it is simple (consists of a monatomic molecule).

Determination of the molar mass of a gas Take a vessel of known volume and put some mass of gas into it. To do this, first pump the gas out of it and weigh it, and then pump the gas in and weigh it again. Then measure the gas pressure in pascals with a thermometer and its temperature. To convert Celsius to , add 273 to them. In order to find the molar mass, by converting the Clapeyron-Mendeleev equation, take the mass of the gas in grams, multiply it by the temperature and the number 8.31, which is universal. Divide the resulting number by the pressure in cubic meters (M = m 8.31 T / (P V)). The result will be the molar mass of the gas in grams per mole.

Related videos

Sources:

• molar masses of substances table

To find the molar mass substances, determine its chemical formula and, using the periodic table of Mendeleev, calculate its molecular mass. It is numerically equal to the molar mass substances in grams per mole. If the mass of one molecule is known substances, convert it to grams and multiply by 6.022 10^23 (Avogadro's number). molar mass gas can be found using the ideal gas equation of state.

You will need

• periodic table, manometer, thermometer, scales.

Instruction

Determination of molar mass by chemical formula. Find the elements in the periodic table of Mendeleev that correspond to the atoms that the molecule consists of substances. If the molecule substances monoatomic, then this will be his. If not, find the atomic number of each element, and add up those masses. The result will be the molar mass substances, expressed in grams per mole.

Molar mass determination substances by the mass of one molecule. In the event that the mass of one molecule is known, convert it to, then multiply by the number of molecules in one mole of any substances, which is 6.022 10^23 (Avogadro's number). Get a molar mass substances in grams per mole.

Determination of the molar mass of a gas. Take a cylinder that can be hermetically sealed with a known volume, which is translated into. Use the pump to pump out the gas from it, and weigh the empty cylinder on the scales. Then fill it with the gas whose molar mass is being measured. Weigh the balloon again. The difference in the masses of the empty and filled with gas cylinder will be the mass of gas, express it in grams.
Using a pressure gauge, measure the gas pressure inside the cylinder, to do this, attach it to the gas injection hole. You can immediately use a cylinder with a built-in pressure gauge to quickly monitor pressure indicators. Measure pressure in pascals.

Wait for some time for the gas inside the cylinder to equalize with the ambient temperature, and measure it with a thermometer. Convert the indicator from degrees Celsius to kelvins, for which add the number 273 to the measured value.
Multiply the mass of the gas by the temperature and the universal gas constant (8.31). Divide the resulting number sequentially by the pressure and volume values ​​​​(M \u003d m 8.31 T / (P V)). The result will be the molar mass of the gas in grams per mole.

Sources:

• determination of molar mass

Molecular weight is the molecular weight, which can also be called the value of the mass of a molecule. The molecular weight is expressed in atomic mass units. If we disassemble the value of the molecular weight in parts, it turns out that the sum of the masses of all the atoms that make up the molecule is its molecular weight. mass. If we talk about units of mass, then mostly all measurements are made in grams.

Instruction

Molecular weight itself is related to the concept of a molecule. But it cannot be said that this condition can be applied only to those where the molecule, for example, hydrogen, is located separately. For cases where the molecules are not separate from the rest, but in close relationship, all the above conditions and definitions are also valid.

To start, to define mass hydrogen, you will need - either, in which hydrogen is composed and from which it can be easily isolated. This can be some kind of alcohol solution or another mixture, some of the components of which, under certain conditions, change their state and easily free the solution from its presence. Find a solution from which you can vaporize the necessary or unnecessary substances by heating. This is the easiest way. Now decide whether you will evaporate a substance that you do not need or whether it will be hydrogen, molecular mass which you plan to measure. If an unnecessary substance evaporates, it's okay that it is not toxic. in the case of the evaporation of the desired substance, you need equipment so that all the evaporation is preserved in the flask.

After you have separated everything unnecessary from the composition, proceed to measurements. For this, Avogadro's number will suit you. It is with its help that you can calculate the relative atomic and molecular mass hydrogen. Find all the options you need hydrogen which are present in any table, determine the density of the resulting gas, as it will come in handy for one of the formulas. Then substitute all the results obtained and, if necessary, change the unit of measurement to , as already mentioned above.

The concept of molecular weight is most relevant when it comes to polymers. It is for them that it is more important to introduce the concept of average molecular weight, due to the heterogeneity of the molecules that make up their composition. Also, by the average molecular weight, one can judge how high the degree of polymerization of a particular substance is.

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Molecular mass is the mass of a molecule of a substance, expressed in atomic units. Often the problem arises: to determine the molecular weight. How can I do that?

Instruction

If you know, then the problem is solved elementarily. All you need is the Periodic Table. For example, you want to find the molecular weight of chloride. Write the formula of the substance: CaCl2. According to the periodic table, establish the atomic mass of each element included in its composition. For calcium, it is (rounded) 40, for (also rounded) - 35.5. Given index 2, find: 40 + 35.5 * 2 \u003d 111 a.m.u. (atomic mass units).

But what about in cases where the exact substance is unknown? Here you can act in different ways. One of the most effective (and at the same time, simple) is the so-called “osmotic pressure method”. It is based on osmosis, which consists in the fact that solvent molecules can penetrate a semi-impermeable, while solute molecules cannot penetrate through it. The value of osmotic pressure can be measured, and it is directly proportional to the concentration of the molecules of the substance under study (that is, their number per unit volume of the solution).

Some people are familiar with the universal Mendeleev-Clapeyron equation, which describes the state of the so-called "ideal gas". It looks like this: PVm = MRT. Van't Hoff's formula is very similar to it: P = CRT, where P is the osmotic pressure, C is the molar concentration of the solute, R is the universal gas constant, T is the temperature in degrees Kelvin. This similarity is not accidental. It was as a result of the work of van't Hoff that it became clear that molecules (or ions) behave as if they were in a gas (with the same volume).

By measuring the value of the osmotic pressure, it is possible to simply calculate the molar concentration: С=P/RT. And then, knowing also the mass of the substance in the solution, find its molecular weight. Suppose it has been experimentally established that the molar concentration of the already mentioned substance is 0.2. At the same time, in a solution of 22.2 grams of this substance. What is its molecular weight? 22.2/0.2 = 111 amu - exactly the same as the previously mentioned calcium chloride.

Related videos

Molecular mass substances is the mass of a molecule, expressed in atomic units and numerically equal to the molar mass. In calculations in chemistry, physics and technology, the calculation of the values ​​​​of the molar mass of various substances is often used.

You will need

• - Mendeleev table;
• - table of molecular weights;
• - table of cryoscopic constant values.

Instruction

Find the desired element in the periodic table. Pay attention to fractional numbers under its sign. For example, O has a cell value of 15.9994. This is the atomic mass of the element. nuclear mass must be multiplied by the index of the element. The index shows how much of the element is contained in the substance.

If a complex is given, then multiply the atomic mass of each element by its index (if there is one atom of one or another element and there is no index, respectively, then multiply by one) and add the resulting atomic masses. For example, water is calculated as follows - MH2O = 2 MH + MO ≈ 2 1 + 16 = 18 a. eat.

Calculate the molar mass using suitable formulas and equate it to the molecular one. Change units from g/mol to a.m.u. Given pressure, volume, absolute Kelvin temperature, and mass, calculate the molar mass gas according to the Mendeleev-Claiperon equation M=(m∙R∙T)/(P∙V), in which M is the molecular () in amu, R is the universal gas constant.

Calculate molar mass according to the formula M=m/n, where m is the mass of any given substances, n - chemical quantity substances. Express Quantity substances through the Avogadro number n=N/NA or using the volume n=V/VM. Plug into the formula above.

Find the molecular mass gas, if only the value of its volume is given. To do this, take a sealed cylinder of known volume and pump out of it

This is the ratio of the mass of the molecule ma of the chemical compound to 1/12 of the mass of the C+12 carbon atom: Mr = ma/(1/12mc) = (12ma)/mc. The relative molecular mass is a dimensionless quantity. The table of D. I. Mendeleev will help determine the relative molecular mass by rounding the values ​​of atomic masses to two to three significant figures. The relative atomic mass is designated Ar, its value is indicated in the periodic table of elements under the chemical sign of the element. The relative molecular mass of a substance is equal to the sum of the relative atomic masses of all elements (including indices). So, how to calculate the relative molecular weight, for example, of water molecules (H2O): Mr (water) \u003d 2Ar (H) + Ar (O) ≈ 2 1 + 16 \u003d 18 The relative molecular weight of water is 18 and this means that the mass of the molecule 18 times more water than 1/12 of the mass of a C+12 atom.

A mixture is two or more substances alternating with each other in space. In this case, the properties of the sweep components remain unchanged. How to find the mass of a mixture? The mass of the mixture is equal to the sum of the masses of the components that make up the mixture. mcm = m1 + m2+….. mi. The density of a mixture ρ(cm) of a mixture consisting of i components and occupying a volume V is determined by the formula: ρ (cm)= m/V =(m1+m2+…+mi)/V = m1/V + m2/V +… + mi/V = ρ1 + ρ2 +…+ ρiwhere m1, m2,………..mi; and ρ1,ρ2,…..ρi are the masses and densities of each of the components of the mixture. The densities of the components of the mixture are determined from reference books. The mass of the mixture m is equal to ρcmV = (ρ1 + ρ2 +…+ ρi)V

Substances are made up of molecules, a molecule is the smallest particle of a given substance. How to determine the mass of a molecule? The mass of one molecule of a substance can be determined by dividing the molar mass of a substance by the number of molecules in one mole, that is, by the Avogardo number. Hence m0 = M/NA. The molar mass is numerically the same as the molecular mass, but differs in dimension. Its dimension is g / mol. For example, the molar mass of water is 18 g / mol. To find out the mass of a water molecule, we divide 18 by the Avogadro number NA \u003d 6.022 × 10 ^ 23: 18 / (6.022 × 10 ^ 23) \u003d 2.99 10^-23. Acting in a similar way, it is easy to find the mass of a molecule of a substance with a known chemical formula.

Substances can exist in several states - solid, liquid and gaseous. Gases are constantly changing shape and volume. How to calculate the mass of a gas? The equation of state of a gas relates the thermodynamic parameters characterizing the gas - pressure p, volume V and temperature Т:рV = (m/M)RT, where m is the gas mass, M is the molar mass, R is the universal gas constant. The mass of the gas can be determined if the state parameters are known: m = pVM/RT. The universal gas constant, denoted R, is 8.314 J/(K.mol) or 1.987 cal/(K.mol). It is a universal constant - a fundamental physical constant.

Solutions are a homogeneous multicomponent system consisting of a solvent, dissolved substances and products of their interaction. How to determine the mass of a solution? You can find the mass of a solution using the formula for finding the mass of a solution: m (solution) \u003d m (substance) + m (solvent). Solutions are found everywhere. For example, a solution of carbon dioxide (CO2) in water is the familiar sparkling water. One of the most important properties of water is its ability to dissolve various substances. Solutions can also be gaseous, but liquid mixtures in chemistry are simply called solutions. Water is the most commonly used inorganic solvent.