Mass fraction of an element in a complex substance - Knowledge Hypermarket. How to find the mass fraction of a substance using the formula Mass fraction of a chemical element in a substance

1. Fill in the gaps in the sentences.

a) In mathematics, a “share” is the relationship of a part to the whole. To calculate the mass fraction of an element, you need to multiply its relative atomic mass by the number of atoms of this element in the formula and divide by the relative molecular mass of the substance.

b) The sum of the mass fractions of all elements that make up the substance is 1 or 100%.

2. Write down mathematical formulas to find the mass fractions of elements if:

a) formula of the substance - P 2 O 5, M r = 2*31+5*16=142
w(P) = 2*31/132 *100% = 44%
w(O) = 5*16/142*100% = 56% or w(O) = 100-44=56.

b) formula of the substance - A x B y
w(A) = Ar(A)*x/Mr(AxBy) * 100%
w(B) = Ar(B)*y / Mr(AxBy) *100%

3. Calculate the mass fractions of elements:

a) in methane (CH 4)

b) in sodium carbonate (Na 2 CO 3)

4. Compare the mass fractions of the indicated elements in the substances and put a sign<, >or = :

5. In a compound of silicon with hydrogen, the mass fraction of silicon is 87.5%, hydrogen 12.5%. The relative molecular weight of the substance is 32. Determine the formula of this compound.

6. The mass fractions of elements in the compound are reflected in the diagram:

Determine the formula of this substance if it is known that its relative molecular weight is 100.

7. Ethylene is a natural stimulator of fruit ripening: its accumulation in fruits accelerates their ripening. The earlier the accumulation of ethylene begins, the earlier the fruits ripen. Therefore, ethylene is used to artificially accelerate fruit ripening. Derive the formula of ethylene if it is known that the mass fraction of carbon is 85.7%, the mass fraction of hydrogen is 14.3%. The relative molecular weight of this substance is 28.

8. Derive the chemical formula of a substance if it is known that

a) w(Ca) = 36%, w(Cl) = 64%

b) w(Na) 29.1%, w(S) = 40.5%, w(O) = 30.4%.

9. Lapis has antimicrobial properties. Previously, it was used to cauterize warts. In small concentrations it acts as an anti-inflammatory and astringent, but can cause burns. Derive the formula of lapis if it is known that it contains 63.53% silver, 8.24% nitrogen, 28.23% oxygen.

Knowing the chemical formula, you can calculate the mass fraction of chemical elements in a substance. element in substance is denoted in Greek. letter “omega” - ω E/V and is calculated using 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) = 2*A r (H) + 1*A r (O) = 2*1 + 1* 16 = 18

2) Calculate the mass fraction of hydrogen in water:

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

Rice. 1. Formulation of the solution to problem 1

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

1) Calculate the relative molecular mass of the substance:

M r (H 3 PO 4) = 3*A r (N) + 1*A r (P) + 4*A r (O) = 3*1 + 1* 31 +4*16 = 98

2) 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 problems and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry, 8th grade” / 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. 8th grade” / 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 education institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§15)

4. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed.V.A. Volodin, Ved. scientific ed. I. Leenson. - M.: Avanta+, 2003.

1. Unified collection of digital educational resources ().

2. Electronic version magazine "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. With. 34-36 No. 3.5 from Workbook in chemistry: 8th grade: to the textbook P.A. Orzhekovsky and others. “Chemistry. 8th grade” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006.

What is mass fraction? For example, 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 either as a percentage or as a fraction.

Where can 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 from the mass of a complex substance

For calculations, use the online Molar mass of a substance calculator with extended data that can be seen if you use an XMPP request.

Calculation of tasks similar to those listed above becomes even simpler, more convenient and more accurate when using this page. Speaking of accuracy. For some reason, in school textbooks the molar masses of elements are rounded to whole 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 there is nothing complicated about it. For the most part, those atomic masses of elements that the calculator uses are sufficient to solve the tasks assigned to determine the mass fractions of elements

But for those pedants :) for whom accuracy is important, 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 problems and how to solve them. Note that despite the fact that they are all heterogeneous, they are essentially based on molar mass substances and mass fractions of elements in this substance

At the beginning of autumn 2017, I added another calculator Mole fractions of a substance and number of atoms, which will help solve problems on 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, we just write the formula and get the result which will be our answer

As has already been said, school textbooks contain rather rough meanings, so don’t be surprised if you see in the answers of paper books Cu = 40%, O = 40%, S = 20%. Let's put it this way" side effects" simplifying school material for students. For real problems, our answer (bot answer) is naturally more accurate.

If we were talking about what to express in shares and not percentages, then we divide the percentages of each of the elements by 100 and get the answer in shares.

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

Let's enter the cryolin formula and get the following data

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

Whether we measure it in grams, 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 turned out to be a typical proportion. You can, of course, use the bot Calculation of proportions and ratios, but this proportion is so simple that we’ll do it manually.

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 school, sometimes you will have to round the mass content of elements in a substance to a whole number, but the answer is actually not very different from the previous one

69*10/210=3.285714

The accuracy down to hundredths is the same.

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

The 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 quantity

answer 20.63 tons of oxygen are in a given mass of substance.

If we add a service symbol to the formula Exclamation point, telling us that the problem is for school purposes (rough rounding is used atomic masses to integers), then we get the following answer:

The proportion will be like this

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

20.64 tons

Something like this:)

Good luck with your calculations!!

On this moment About 120 different chemical elements are known, of which no more than 90 can be found in nature. The variety of different chemical substances around us is incommensurably greater than this number.
This is due to the fact that very rarely chemical substances consist of individual, unrelated atoms of chemical elements. Under normal conditions, only a small number of gases called noble gases have this structure - helium, neon, argon, krypton, xenon and radon. Most often, chemical substances do not consist of isolated atoms, but of their combinations into various groups.
That is, the atoms of most chemical elements are able to bond with each other. Most often, this results in molecules - particles that are groups of two or more atoms. For example, the chemical substance hydrogen consists of hydrogen molecules, which are formed from atoms as follows:

Figure 3. Formation of a hydrogen molecule

Atoms of different chemical elements can also form bonds with each other; for example, when an oxygen atom interacts with two hydrogen atoms, a water molecule is formed:

Figure 4. Formation of a water molecule

Since it is inconvenient to draw atoms of chemical elements and label them each time, chemical formulas were invented to reflect the composition of molecules. So, for example, the formula for molecular hydrogen is written as H 2, where the number 2, written in subscript to the right of the hydrogen atom symbol, means the number of atoms of this type in the molecule. Thus, the formula of water can be written as H 2 O. The unit, which should indicate the number of oxygen atoms in the molecule, according to the rules accepted in chemistry, is not written. Numbers indicating the number of atoms in one molecule are called indices.
Let's look at a few more examples of chemical formulas of substances. Thus, the formula of ammonia is written as NH 3, which indicates that each ammonia molecule consists of one nitrogen atom and three hydrogen atoms.
There are often molecules in which several identical groups of atoms can be counted. For example, from the formula of aluminum sulfate Al 2 (SO 4) 3, we can conclude that the molecule of this substance contains two groups of SO 4 atoms.
Thus, the chemical formulas of substances unambiguously characterize both their qualitative and quantitative composition.
From all of the above, the law of constancy of the composition of matter, established back in 1808 by the French scientist Joseph Louis Proust, logically follows, and it sounds as follows:

Any pure chemical substance has a constant qualitative and quantitative composition, regardless of the method of obtaining this substance.

Since any chemical substance is a collection of molecules of the same composition, this leads to the fact that the proportions between the atoms of chemical elements in any portion of the substance are the same as in one molecule of this substance. All differences in chemical properties substances depend on the quantitative and qualitative composition of the molecules and, in addition, on the order of connections between atoms, if possible.
So you can give following definition molecule term:

A molecule is the smallest particle of a chemical substance that has its chemical properties.

Similar to relative atomic mass, there is also such a thing as relative molecular mass Mr:

The relative molecular mass (M r) of a substance is the ratio of the mass of one molecule of this substance to one twelfth of the mass of one carbon atom (1 atomic mass unit).

Thus, it is obvious that the relative molecular mass is the sum of the relative atomic masses of the elements, each of which is multiplied by the number of atoms of that particular type in one molecule. So, for example, the relative molecular weight of the nitric acid molecule HNO 3 consists of the relative atomic mass of hydrogen, the relative atomic mass of nitrogen and three relative atomic masses of oxygen:

To describe the qualitative and quantitative composition of a substance, the concept of mass fraction of a chemical element is used w(X).

Where can 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 from the mass of a complex substance

For calculations, use the online Molar mass of a substance calculator with extended data that can be seen if you use an XMPP request.

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

Examples

Calculate the mass fraction of elements in copper sulfate CuSO 4

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

As has already been said, school textbooks contain rather rough meanings, so don’t be surprised if you see in the answers of paper books Cu = 40%, O = 40%, S = 20%. These are, let’s say, “side effects” of simplifying school material for students. For real problems, our answer (bot answer) is naturally more accurate.

If we were talking about what to express in shares and not percentages, then we divide the percentages of each of the elements by 100 and get the answer in shares.

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

Let's enter the cryolin formula and get the following data

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

Whether we measure it in grams, 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 turned out to be a typical proportion. You can, of course, use the bot Calculation of proportions and ratios, but this proportion is so simple that we’ll do it manually.

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 school, sometimes you will have to round the mass content of elements in a substance to a whole number, but the answer is actually not very different from the previous one

69*10/210=3.285714

The accuracy down to hundredths is the same.

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

The 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 quantity

answer 20.63 tons of oxygen are in a given mass of substance.

If we add an exclamation mark to the formula, which tells us that the problem is for school purposes (rough rounding of atomic masses to whole numbers is used), then we will get the following answer.