How to find a mass fraction of a substance in the compound. Calculation of the mass fraction of the element or substance

What is a mass fraction? For example, mass fraction chemical element - this is the relationship of the element to the mass of all substances. Mass fraction can be expressed as percentage and fractions.

Where can the mass proportion be used?

Here are some of the directions:

Determination of the elementary composition of a complex chemical

Finding the mass of element by weight of complex substance

For calculations, the calculator is a molar mass of the substance online with advanced data that can be seen if you use an XMPP query.

Calculation of such tasks, which are indicated above, when this page is accepted, it becomes even easier, more convenient and more accurate. By the way about accuracy. In school textbooks for some reason, the molar masses of the elements are rounded to the whole values, which is to solve school tasks This is quite useful, although in fact the molar masses of each chemical element are periodically indulcable.

Our calculator does not seek to show high accuracy (above 5 characters after the comma), although there is nothing complicated. For the most part, the atomic masses of the elements that use the calculator are sufficient to solve the tasks to determine the mass fractions of the elements

But for those pedants :), which is important accuracy, I would like to recommend the link Atomic Weights and Isotopic Compositions for All Element Sin which all chemical elements are displayed, their relative atomic masses, as well as the masses of all isotopes of each of the element.

That's all I would like to say. Now we will consider specific tasks And how to solve them. Note that despite the fact that they are all heterogeneous, they are in their essence rely on molar Mass substances and mass fractions of elements in this substance

At the beginning of the fall of 2017, I added another calculator of the molar shares of the substance and the number of atoms that will help solve the masses for the mass clean substance In a complex substance, the amount of mole in the substance and in each element, as well as the number of atoms / molecules in the substance.

Examples

Calculate mass share Elements in copper Sulfate CUSO 4

The request is very simple, just write the formula and get the result that will be our answer

As already mentioned in school textbooks there are enough wedlized values, so do not be surprised if you see in the answers of paper books Cu \u003d 40%, O \u003d 40%, S \u003d 20%.This will be said to be the "side effects" of simplifying school material, for students. For real tasks, our answer (Bot's response) is natural more accurate.

If it came to express in fractions and not percentages, then we divide the percentages of each of the elements by 100 and we receive an answer in fractions.

How much sodium is contained in 10 tons of cryoline Na3?

We introduce a cryoline formula and obtain the following data

From the data obtained, we see that in 209,9412 the quantities of the substance contains 68,96931 amounts of sodium.

In grams, we will measure it, in kilograms or tons for the ratio it does not change.

Now it remains to build another match where we have 10 tone of the starting material and an unknown amount of sodium

It turned out a typical proportion. Of course, it is possible to take advantage of the calculation of proportions and ratios, but this proportion is so simple that we will do it with handles.

209,9412 refers to 10 (tons) as 68,96391 to an unknown number.

Thus, the amount of sodium (in tons) in cryoline will be 68.96391 * 10 / 209.9412 \u003d 3.2849154906231 tons of sodium.

Again for school sometimes they will have to round up to an integer mass content of elements in the substance, but the answer is actually not much different from the previous

69*10/210=3.285714

Accuracy to hundredths coincides.

Calculate how much oxygen is contained in 50 tons of calcium phosphate CA3 (PO4) 2?

Mass fractions of a given substance are the following

The same proportion as in the previous problem 310.18272 refers to 50 (tons) as well as 127.9952 to an unknown value

the response of 20.63 tons of oxygen is in a given mass of the substance.

If we add an exclamation mark to the formula to the formula that tells us that the task of school (coarse rounding of atomic masses up to integers), then the answer is next.

What is a mass fraction in chemistry? Do you know the answer? How to find a mass fraction of an element in substance? The process of calculation itself is not so complicated at all. Do you still have difficulty in such tasks? Then lucky you smiled, you found this article! Interesting? Then read, now you will understand everything.

What is a mass fraction?

So, for a start, find out what is a mass fraction. How to find a mass fraction of the element in the substance, any chemist will answer, as they often use this term when solving tasks or during stay in the laboratory. Of course, after all, its calculation is their everyday task. To obtain a certain amount of a substance in the laboratory, where the accurate calculation is very important and all possible options Outcome reactions, you need to know only a couple simple formulas And understand the essence of the mass fraction. Therefore, this topic is so important.

This term is indicated by the "W" symbol and read as "Omega". It expresses the ratio of the mass of this substance to the total mass of the mixture, solution or molecule, expressed by a fraction or percentage. The formula for calculating the mass fraction:

w \u003d m substance / m mixtures.

We transform the formula.

We know that m \u003d n * m, where m is a mass; n is the amount of substance expressed in mole measurement units; M is a molar mass of a substance expressed in gram / mole. The molar mass is numerically equal to molecular. Only molecular weight is measured in atomic units Mass or a. E. m. Such a unit of measurement is equal to one twelfth share of carbon kernel 12. The molecular weight value can be found in the Mendeleev table.

The amount of substance N of the desired object in this mixture is equal to the index multiplied by the coefficient at this compound, which is very logical. For example, to calculate the number of atoms in the molecule, it is necessary to know how many atoms of the desired substance are in 1 molecule \u003d index, and multiply this number to the amount of molecules \u003d coefficient.

You should not be afraid of such bulky definitions or formulas, they trace a certain logic, which, which, you can even learn the formulas themselves. The molar mass M is equal to the sum of the atomic mass A R of this substance. Recall that the atomic mass is the mass of 1 of the atom of the substance. That is, the source formula of the mass fraction:

w \u003d (n substances * m substance) / m mixture.

From this we can conclude that if the mixture consists of one substance, the mass fraction of which should be calculated, then w \u003d 1, since the mass of the mixture and the mass of the substance coincide. Although a priori mixture cannot consist of a single substance.

So, with theory figured out, but how to find a mass fraction of an element in a substance in practice? Now we will show everything and tell.

Checking the learned material. Task of light level

Now we will analyze two tasks: light and medium level. Read more!

It is necessary to learn the mass fraction of iron in the molecule of the FESO 4 * 7 H 2 Oh. How to solve this task? Consider the decision further.

Decision:

Take 1 mole FESO 4 * 7 H 2 O, then we learn the amount of iron, multiplying the iron coefficient on its index: 1 * 1 \u003d 1. Dan 1 mol iron. We learn its mass in the substance: from the value in the Mendeleev table, it can be seen that the atomic weight of iron is 56 a. e. m. \u003d 56 grams / mole. In this case, a r \u003d m. Therefore, m iron \u003d n * m \u003d 1 mol * 56 grams / mol \u003d 56 g.

Now you need to find a mass of the whole molecule. It is equal to the sum of the masses of the source substances, that is, 7 mol of water and 1 mol of iron sulfate.

m \u003d (n water * m water) + (n iron sulfate * m iron sulfate) \u003d (7 mol * (1 * 2 + 16) gram / mol) + (1 mol * (1 mol * 56 grams / mol + 1 mol * 32 grams / mol + 4 mol * 16 grams / mol) \u003d 126 + 152 \u003d 278

It remains only to divide the mass of iron on the mass of the connection:

w \u003d 56g / 278 g \u003d 0.20143885 ~ 0.2 \u003d 20%.

Answer: 20%.

The object of average

I decide a more complex task. In 500 g of water dissolved 34 g of calcium nitrate. It is necessary to find a mass fraction of oxygen in the resulting solution.

Decision

Since the interaction of Ca (NO 3) 2 with water, there is only a dissolution process, and the reaction products are not released from the solution, the mass of the mixture is equal to the sum of the mass of calcium and water nitrate.

We need to find a mass fraction of oxygen in solution. We draw attention to the fact that oxygen is contained both in the dissolved substance and in the solvent. Find the number of the desired element in the water. To do this, we calculate the mole of water according to the formula n \u003d m / m.

n water \u003d 500 g / (1 * 2 + 16) gram / mol \u003d 27.7777-328 mol

From the formula of the water H 2 O we find that the amount of oxygen \u003d the amount of water, that is, 28 mol.

Now we will find the amount of oxygen in dissolved Ca (NO 3) 2. For this we know the amount of substance itself:

n Ca (NO3) 2 \u003d 34 g / (40 * 1 + 2 * (14 + 16 * 3)) gram / mol≈0.2 mol.

n Ca (NO3) 2 refers to N o as 1 to 6, which follows from the compound formula. So, N o \u003d 0.2 mol * 6 \u003d 1.2 mol. The total amount of oxygen is 1.2 mol + 28 mol \u003d 29.2 mol

m o \u003d 29.2 mol * 16 grams / mol \u003d 467.2

m solution \u003d M water + m Ca (NO3) 2 \u003d 500 g + 34 g \u003d 534

It remains only to calculate the mass fraction of the chemical element in the substance:

w o \u003d 467.2 g / 534 g≈0.87 \u003d 87%.

Answer: 87%.

We hope that we clearly explained to you how to find a mass fraction of the element in the substance. This topic is not at all difficult if it is good to deal well. We wish you good luck and success in future endeavors.

Mass fraction Call the ratio of the mass of this component M (x) to the mass of the entire solution M (RR). Mass fraction is indicated by the symbol ω (omega) and expressed in the fractions of the unit or in percent:

ω (x) \u003d m (x) / m (p-ra) (in the fractions of the unit);

ω (x) \u003d m (x) 100 / m (p-ra) (in percent).

The molar concentration is called the amount of dissolved substance in 1 liter of solution. It is denoted by a symbol with (x) and measured in mol / l:

c (x) \u003d n (x) / v \u003d m (x) / m (x) V.

In this formula N (x) - the amount of substance X contained in solution, M (x) is the molar mass of the substance H.

Consider several typical tasks.

Determine the mass of sodium bromide contained in 300 g of a 15% solution.

Decision.
The mass of sodium bromide is determined by the formula: M (NaBr) \u003d Ω M (RR) / 100;
m (nabr) \u003d 15 300/100 \u003d 45
Answer: 45 g.

2. The mass of potassium nitrate, which should be dissolved in 200 g of water to obtain an 8% solution, is equal to ______ (response round up to an integer.)

Decision.
Let M (Kno 3) \u003d x r, then m (p-ra) \u003d (200 + x)
Mass fraction of potassium nitrate in solution:
ω (KNO 3) \u003d x / (200 + x) \u003d 0.08;
x \u003d 16 + 0.08x;
0.92x \u003d 16;
x \u003d 17.4.
After rounding x \u003d 17 g
Answer: 17 g.

3. The mass of calcium chloride, which should be added to 400 g of a 5% solution of the same salt to double its mass fraction, is equal to ______. (Record the answer with an accuracy of the tenths.)

Decision.
The mass CaCl 2 in the initial solution is:
m (CaCl 2) \u003d Ω M (RR);
M (CaCl 2) \u003d 0.05 400 \u003d 20 g
The mass fraction of CaCl 2 in the final solution is ω 1 \u003d 0.05 2 \u003d 0.1.
Let the mass of CaCl 2, which should be added to the original solution, is equal to x
Then the mass of the final solution M 1 (p-ra) \u003d (400 + x)
Mass fraction of CaCl 2 in the final solution:

Deciding this equation, we get x \u003d 22.2 g.
Answer: 22.2 g

4. The mass of alcohol, which you need to evaporate from 120 g of a 2% alcohol solution of iodine to increase its concentration to 5%, is equal to _____________. (Answer write up to the tenths.)

Decision.
We define the weight of iodine in the starting solution:
M (I 2) \u003d Ω M (RR);
M (i 2) \u003d 0.02 120 \u003d 2.4 g,
After evaporation, the mass of the solution was equal to:
M 1 (p-ra) \u003d M (i 2) / Ω 1
M 1 (p-ra) \u003d 2.4 / 0.05 \u003d 48
In terms of mass differences, we find a mass of evaporated alcohol: 120-48 \u003d 72
Answer: 72 g.

5. The mass of water that needs to be added to 200 g of a 20% sodium bromide solution to obtain a 5% solution is equal to _________ (response rounded to an integer.)

Decision.
We define the mass of sodium bromide in the source solution:
m (nabr) \u003d Ω M (RR);
M (nabr) \u003d 0.2 200 \u003d 40 g
Let the mass of water that need to be added to dilute the solution is x g, then by the condition of the problem:

From here we get x \u003d 600 g
Answer: 600

6. The mass fraction of sodium sulfate in the solution obtained by mixing 200 g of 5% and 400 g of 10% solutions of Na 2 SO 4 is equal to _____________%. (Answer round up to the tenths.)

Decision.
We define the mass of sodium sulfate in the first source solution:
M 1 (Na 2 SO 4) \u003d 0.05 200 \u003d 10 g
We define the mass of sodium sulfate in the second source solution:
M 2 (Na 2 SO 4) \u003d 0.1 400 \u003d 40 g.
We define the mass of sodium sulfate in the final solution: M (Na 2 SO 4) \u003d 10 + 40 \u003d 50 g.
We define the mass of the final solution: M (p-ra) \u003d 200 + 400 \u003d 600 g.
We define the mass fraction of Na 2 SO 4 in the final solution: 50/600 \u003d 8.3%
Answer: 8,3%.

In addition to solving problems for solutions:

The "Rule of the Cross" call the diagonal scheme of the mixing rules for cases with two solutions.

http://pandia.ru/text/78/476/images/image034_1.jpg "alt \u003d" "width \u003d" 400 "height \u003d" 120 "\u003e
Mass of one part: 300/50 \u003d 6 g.
Then
M1 \u003d 6 15 \u003d 90 g ,.
M2 \u003d 6 35 \u003d 210

You need to mix 90 g of 60% solution and 210 g 10% solution.

What is a mass fraction? For example, the mass fraction of the chemical element is the relationship of the element to the mass of the whole substance. Mass fraction can be expressed as percentage and fractions.

Where can the mass proportion be used?

Here are some of the directions:

Determination of the elementary composition of a complex chemical

Finding the mass of element by weight of complex substance

For calculations, the calculator is a molar mass of the substance online with advanced data that can be seen if you use an XMPP query.

Calculation of such tasks, which are indicated above, when this page is accepted, it becomes even easier, more convenient and more accurate. By the way about accuracy. In school textbooks, for some reason, the molar masses of the elements are rounded to the whole values \u200b\u200bthat it is quite useful to solve school tasks, although in fact the molar masses of each chemical element are periodically indulging.

That's all I would like to say. Now we will consider specific tasks and how to solve them. Note that despite the fact that they are all heterogeneous, they are in their essence rely on the molar mass of the substance and the mass fractions of the elements in this substance

At the beginning of the fall of 2017, I added another calculator of the molar shares of the substance and the number of atoms, which will help solve problems on the mass of pure substance in the complex substance, the amount of mol in the substance and in each element, as well as the number of atoms / molecules in the substance.

Examples

Calculate the mass proportion of elements in copper CUSO 4 sulfate