The formula for finding the mass fraction of a solution in chemistry. How to find the mass fraction of an element in a substance? What it is? Finding the mass fraction of a chemical element

Knowing the chemical formula, you can calculate the mass fraction of chemical elements in a substance. element in substances is denoted by the Greek. the letter "omega" - ω E / V and is calculated by the formula:

where k is the number of atoms of this element in the molecule.

What is the mass fraction of hydrogen and oxygen in water (H 2 O)?

Solution:

M r (H 2 O) \u003d 2 * A r (H) + 1 * A r (O) \u003d 2 * 1 + 1 * 16 \u003d 18

2) Calculate the mass fraction of hydrogen in water:

3) Calculate the mass fraction of oxygen in water. Since the composition of water includes atoms of only two chemical elements, the mass fraction of oxygen will be equal to:

Rice. 1. Formulation of the solution of problem 1

Calculate the mass fraction of elements in the substance H 3 PO 4.

1) Calculate the relative molecular weight of the substance:

M r (H 3 RO 4) \u003d 3 * A r (H) + 1 * A r (P) + 4 * A r (O) \u003d 3 * 1 + 1 * 31 + 4 * 16 \u003d 98

2) We calculate the mass fraction of hydrogen in the substance:

3) Calculate the mass fraction of phosphorus in the substance:

4) Calculate the mass fraction of oxygen in the substance:

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.

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. 34-36)

3. Chemistry: 8th grade: textbook. for general institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§15)

4. 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" ().

4. Video lesson on the topic “Mass fraction chemical element in matter" ().

Homework

1. p.78 No. 2 from the textbook "Chemistry: 8th grade" (P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M .: AST: Astrel, 2005).

2. from. 34-36 №№ 3.5 from Workbook in chemistry: 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.

What is a mass fraction? For example, the mass fraction of a chemical element is the ratio of the mass of the element to the mass of the entire substance. The mass fraction can be expressed both as a percentage and fractions.

Where can the mass fraction be used?

Here are some of the directions:

Determination of the elemental composition of a complex chemical substance

Finding the mass of an element by the mass of a complex substance

For calculations, the Molar Mass of a Substance calculator is used online with extended data that can be seen if you use an XMPP query.

The calculation of similar tasks that are indicated above, when using this page, becomes even easier, more convenient and more accurate. Speaking of accuracy. In school textbooks, for some reason, the molar masses of elements are rounded to integer values, which is quite useful for solving school problems, although in fact the molar masses of each chemical element are periodically adjusted.

Our calculator does not strive to show high accuracy (above 5 decimal places), although this is not difficult. For the most part, those atomic masses of elements that use the calculator are sufficient to solve the tasks set for determining the mass fractions of elements

But for those pedants :) who care about accuracy, I would like to recommend the link Atomic Weights and Isotopic Compositions for All Elements which displays all the chemical elements, their relative atomic masses, as well as the masses of all isotopes of each element.

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

At the beginning of autumn 2017, I added another calculator Mole fractions of matter and the number of atoms, which will help solve problems for the mass of a pure substance in a complex substance, the number of moles in a substance and in each element, as well as the number of atoms / molecules in a substance.

Examples

Calculate the mass fraction of elements in copper sulfate CuSO 4

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

As already mentioned in school textbooks, there are rather rough values, so do not be surprised if you see in the answers of paper books Cu = 40%, O = 40%, S = 20%. This is, let's say, "side effects" of simplification school material, for students. For real problems, our answer (the bot's answer) is naturally more accurate.

If it was about what to express in fractions and not percentages, then we divide the percentages of each of the elements by 100 and get the answer in fractions.

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

We introduce the cryolin formula and get the following data

From the data obtained, we see that 209.9412 amounts of a substance contain 68.96931 amounts of sodium.

Whether we measure it in grams, in kilograms or tons, nothing changes for the ratio.

Now it remains to build another correspondence where we have 10 tons of the original substance and an unknown amount of sodium

This is a typical proportion. Of course, you can use the Calculation of proportions and ratios bot, but this proportion is so simple that we will do it with handles.

209.9412 is to 10 (tons) as 68.96391 is to an unknown number.

Thus, the amount of sodium (in tons) in cryolin will be 68.96391*10/209.9412=3.2849154906231 tons of sodium.

Again, for the school, sometimes it will be necessary to round up to an integer the mass content of elements in a substance, but the answer is actually not very different from the previous one.

69*10/210=3.285714

Accuracy to hundredths is the same.

Calculate how much oxygen is contained in 50 tons of calcium phosphate Ca3(PO4)2?

Mass fractions of a given substance are as follows

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

answer 20.63 tons of oxygen is in a given mass of matter.

If we add an exclamation mark to the formula, telling us that the task is school (rough rounding of atomic masses to integers is used), then we get the following answer.

Where can the mass fraction be used?

Here are some of the directions:

Determination of the elemental composition of a complex chemical substance

Finding the mass of an element by the mass of a complex substance

For calculations, the Molar Mass of a Substance calculator is used online with extended data that can be seen if you use an XMPP query.

Examples

Calculate the mass fraction of elements in copper sulfate CuSO 4

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

As already mentioned in school textbooks, there are rather rough values, so do not be surprised if you see in the answers of paper books Cu = 40%, O = 40%, S = 20%. This is, let's say, the "side effects" of simplifying school material for students. For real problems, our answer (the bot's answer) is naturally more accurate.

If it was about what to express in fractions and not percentages, then we divide the percentages of each of the elements by 100 and get the answer in fractions.

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

We introduce the cryolin formula and get the following data

From the data obtained, we see that 209.9412 amounts of a substance contain 68.96931 amounts of sodium.

Whether we measure it in grams, in kilograms or tons, nothing changes for the ratio.

Now it remains to build another correspondence where we have 10 tons of the original substance and an unknown amount of sodium

This is a typical proportion. Of course, you can use the Calculation of proportions and ratios bot, but this proportion is so simple that we will do it with handles.

209.9412 is to 10 (tons) as 68.96391 is to an unknown number.

Thus, the amount of sodium (in tons) in cryolin will be 68.96391*10/209.9412=3.2849154906231 tons of sodium.

Again, for the school, sometimes it will be necessary to round up to an integer the mass content of elements in a substance, but the answer is actually not very different from the previous one.

69*10/210=3.285714

Accuracy to hundredths is the same.

Calculate how much oxygen is contained in 50 tons of calcium phosphate Ca3(PO4)2?

Mass fractions of a given substance are as follows

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

answer 20.63 tons of oxygen is in a given mass of matter.

If we add an exclamation mark to the formula, telling us that the task is school (rough rounding of atomic masses to integers is used), then we get the following answer:

The proportion will be like this

310 refers to 50 (tons) as well as 128 to an unknown quantity. And the answer

20.64 tons

Something like this:)

Good luck with your calculations!!

TOPIC OF THE LESSON: Mass fraction of a chemical element in a compound.

THE PURPOSE OF THE LESSON: To teach how to calculate the mass fraction of elements in a compound according to the formula of the compound and establish the chemical formula of a complex substance from known mass fractions of chemical elements.

Basic concepts. Mass fraction of a chemical element.

Planned learning outcomes

subject. To be able to calculate the mass fraction of an element in a compound according to its formula and establish the chemical formula of a complex substance from known mass fractions of chemical elements.

Metasubject . To form the ability to establish analogies, use algorithms to solve educational and cognitive problems.

The main activities of students. Calculate the mass fraction of an element in a compound using its formula. Establish the chemical formula of a complex substance according to known mass fractions of chemical elements.

Lesson structure

I. Organizational stage

II. Updating of basic knowledge

III. Learning new material

IV. Consolidation. Summing up the lesson

V. Homework

During the classes

Organizing time.

Checking homework.

Updating of basic knowledge.

Give definitions: relative atomic mass, relative molecular mass.

In what units can relative atomic mass be measured?

In what units can relative molecular weight be measured?

Learning new material.

Work with the textbook. Workbook.

Guys, let's say we have a substance - sulfuric acidH 2 SO 4,

Can we find out which atoms are part of the compound.

What about their number?

And in what mass ratio do they combine?

Calculation of chemical mass ratios

elements in complex matter. (page 51)

And how can you find out in what mass ratios the elements are connected in a compound whose formula isH 2 SO 4 ?

m(H): m(S): m(O)= 2*2 + 32 + 16*4= 2:32:64 = 1:16:32.

1 + 16 + 32 \u003d 49, that is, 49 mass parts of sulfuric acid, contain 1 mass part of hydrogen, 16 mass parts of sulfur, 32 mass parts of oxygen.

Guys, what do you think, can we calculate the proportion of each element in the compound?

Today we will get acquainted with the new concept of the mass fraction of an element in a compound.

W- mass fraction of the element in the compound.

nis the number of atoms of the element.

Mr- relative molecular weight.

Calculation of mass fractions of chemical elements

in complex matter. (RT)

1. Study the algorithm for calculating the mass fraction of an element in a compound.

Task #1 (RT)

Output chemical formulas if mass fractions of chemical elements are known,

included in this substance. (RT)

2. Study the algorithm for calculating the mass fraction of an element in a compound.

Task #5 (RT)

Consolidation of the studied material.

RT p. 25 No. 2.

RT p. 27 No. 6.

Summing up the lesson.

What new concepts did you learn in class today?

Independent work.

Homework:

study §15 pp. 51 - 53;
answer questions No. 3,4,7 pp. 53-54 (in writing).

P list of used literature.

Textbook. Chemistry grade 8. ed. G.E. Rudzitis, F.G. Feldman. Publishing house "Enlightenment", 2014.

Chemistry workbook. ed. Borovskikh T.A.

What is mass fraction in chemistry? Do you know the answer? How to find the mass fraction of an element in a substance? The calculation process itself is not so complicated at all. Are you still having trouble doing this kind of work? Then luck smiled at you, you found this article! Interesting? Then read on, now you will understand everything.

What is mass fraction?

So, first, let's find out what a mass fraction is. How to find the mass fraction of an element in a substance, any chemist will answer, since they often use this term when solving problems or during their stay in the laboratory. Of course, because its calculation is their daily task. To obtain a certain amount of a particular substance in laboratory conditions, where accurate calculation is very important and all possible options outcome of reactions, you need to know just a couple simple formulas and understand the essence of the mass fraction. That is why this topic is so important.

This term is denoted by the symbol “w” and is read as “omega”. It expresses the ratio of the mass of a given substance to the total mass of a mixture, solution or molecule, expressed as a fraction or as a percentage. The formula for calculating the mass fraction:

w = m substances / m mixtures.

Let's transform the formula.

We know that m=n*M, where m is the mass; n is the amount of substance, expressed in units of mole; M is the molar mass of the substance, expressed in grams/mol. The molar mass is numerically equal to the molecular mass. Only molecular weight is measured in atomic mass units or a. e. m. Such a unit of measurement is equal to one twelfth of the mass of the carbon nucleus 12. The value of the molecular weight can be found in the periodic table.

The amount of substance n of the desired object in a given mixture is equal to the index multiplied by the coefficient for this compound, which is very logical. For example, to calculate the number of atoms in a molecule, you need to find out how many atoms of the desired substance are in 1 molecule = index, and multiply this number by the number of molecules = coefficient.

You should not be afraid of such cumbersome definitions or formulas, they trace a certain logic, having understood which, you can not even learn the formulas themselves. The molar mass M is equal to the sum of the atomic masses A r of the given substance. Recall that atomic mass is the mass of 1 atom of a substance. That is, the original mass fraction formula:

w = (n substances *M substances)/m mixtures.

From this we can conclude that if the mixture consists of one substance, the mass fraction of which must be calculated, then w = 1, since the mass of the mixture and the mass of the substance are the same. Although a mixture a priori cannot consist of one substance.

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

Checking learned material. Easy level challenge

Now we will analyze two tasks: easy and medium level. Read on!

It is necessary to find out the mass fraction of iron in the molecule of ferrous sulfate FeSO 4 * 7 H 2 O. How to solve this problem? Let's look at the solution next.

Solution:

Let's take 1 mol of FeSO 4 * 7 H 2 O, then we will find out the amount of iron by multiplying the coefficient of iron by its index: 1*1=1. Given 1 mole of iron. We find out its mass in matter: from the value in the periodic table, it can be seen that the atomic mass of iron is 56 a.u. e.m. = 56 grams / mol. IN this case A r =M. Therefore, m iron \u003d n * M \u003d 1 mol * 56 grams / mol \u003d 56 g.

Now we need to find the mass of the entire molecule. It is equal to the sum of the masses of the starting materials, that is, 7 mol of water and 1 mol of ferrous sulfate.

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

It remains only to divide the mass of iron by the mass of the compound:

w=56g/278g=0.20143885~0.2=20%.

Answer: 20%.

Intermediate task

Let's solve a more difficult problem. 34 g of calcium nitrate are dissolved in 500 g of water. You need to find the mass fraction of oxygen in the resulting solution.

Solution

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

We need to find the mass fraction of oxygen in the solution. Note that oxygen is contained in both the solute and the solvent. Find the amount of the desired element in water. To do this, we calculate the mole of water according to the formula n=m/M.

n water \u003d 500 g / (1 * 2 + 16) gram / mol \u003d 27.7777≈28 mol

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

Now let's find the amount of oxygen in dissolved Ca(NO 3) 2 . To do this, we find out the amount of the 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 formula of the compound. Hence, n O = 0.2 mol * 6 = 1.2 mol. The total amount of oxygen is 1.2 mol + 28 mol = 29.2 mol

m O \u003d 29.2 mol * 16 grams / mol \u003d 467.2 g.

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

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

w O =467.2 g / 534 g≈0.87=87%.

Answer: 87%.

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