How to calculate the mass fraction of the element in the connection. Calculation of the mass fraction of the element or substance

1. Fill asleep in sentences.

a) in mathematics "share" - these are relatives to the whole. To calculate the mass fraction of the element, it is necessary to multiply its relative atomic mass by the number of atoms of this element in the formula and divided into relative molecular weight of the substance.

b) The sum of the mass fractions of all elements included in the substance is 1 or 100%.

2. Record mathematical formulas for finding mass fractions of elements if:

a) formula of substance - P 2 O 5, M R \u003d 2 * 31 + 5 * 16 \u003d 142
w (p) \u003d 2 * 31/132 * 100% \u003d 44%
w (O) \u003d 5 * 16/142 * 100% \u003d 56% or W (O) \u003d 100-44 \u003d 56.

b) the substance formula - A x b y
w (a) \u003d Ar (A) * X / MR (AXBY) * 100%
w (B) \u003d Ar (B) * Y / MR (Axby) * 100%

3. Calculate the mass fractions of the elements:

a) in methane (CH 4)

b) in sodium carbonate (Na 2 CO 3)

4. Compare the mass fractions of the specified elements in the substances and put a sign<, > or \u003d:

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

6. Mass stakes 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 fruit ripening stimulator: its savings will accelerate their maturation. The earlier ethylene accumulation begins, the earlier the fruits ripen. Therefore, ethylene is used for artificial acceleration of ripening fruit. Output the formula of ethylene, if it is known that the mass fraction of carbon is 85.7%, the mass fraction of hydrogen -14.3%. The relative molecular weight of this substance is 28.

8. Output the chemical formula of substance, if it is known that

a) w (Ca) \u003d 36%, W (CL) \u003d 64%

b) W (Na) 29.1%, W (S) \u003d 40.5%, W (O) \u003d 30.4%.

9. Lapis has antimicrobial properties. His earlier used to blame warts. In unfinished concentrations, it acts as anti-inflammatory and astringent bodies, but can cause burns. Output Lyapis formula, if it is known that 63.53% of silver is included in its composition, 8.24% nitrogen, 28.23% oxygen.

The concept of "share" is probably familiar to you.

For example, a piece of watermelon, depicted in the figure, is one quarter of the whole watermelon, that is, its share is 1/4 or 25%.

To better understand what is a mass proportion, imagine a kilogram of sweets (1000g), which Mom bought its three children. From this kilogram, the youngest child got half all the candies (unfairless!). Senior - only 200g, and the average - 300g.

So the mass proportion of candies in the younger child will be half, or 1/2 or 50%. The middle child will be 30%, and the older is 20%. It should be emphasized that the mass fraction may be a dimensionless value (quarter, half, third, 1/5, 1/6, etc.), and can be measured as a percentage (%). When solving the calculated tasks, the mass share is better to translate into a dimensionless value.

Mass fraction of substance in solution

Any solution consists of a solvent and solute substance. Water is the most common inorganic solvent. Organic solvents can be alcohol, acetone, diethyl ether, etc. If the solvent is not specified in the task condition, the solution is considered water.

The mass fraction of the dissolved substance is calculated by the formula:

$ \\ omega_ \\ Text (V-BA) \u003d \\ DFRAC (M_ \\ TEXT (V-BA)) (M_ \\ Text (RR)) (\\ CDOT 100 \\%) $

Consider examples of solving problems.

How many grams of sugar and water must be taken to prepare 150g 10% sugar solution?

Decision

m (p-ra) \u003d 150g

$ \\ omega $ (sugar) \u003d 10% \u003d 0.1

m (sugar) \u003d?

m (sugar) \u003d $ \\ omega \\ textrm ((sugar)) \\ Cdot M (P-PA) \u003d 0.1 \\ CDot 150 \\ TextRM (g) \u003d 15 \\ TextRM (g) $

m (water) \u003d m (p-ra) - m (sugar) \u003d 150g - 15g \u003d 135g.

Answer: You need to take 15g sugar and 135g water.

A solution of 350 ml. and a density of 1, 142 g / ml contains 28 g of sodium chloride. Find a mass fraction of salt in solution.

Decision

V (p-ra) \u003d 350 ml.

$ \\ rho $ (p-ra) \u003d 1,142 g / ml

$ \\ omega (NaCl) $ \u003d?

m (p-ra) \u003d V (R-ra) $ \\ cdot \\ rho $ (p-ra) \u003d 350 ml $ \\ cdot $ 1,142 g / ml \u003d 400g

$ \\ omega (NaCl) \u003d \\ DFRAC (M (NaCl)) (M \\ TextRM ((R-RA))) \u003d \\ DFRAC (28 \\ TextRM (g)) (400 \\ TextRM (g)) \u003d 0.07 $ \u003d 7%

Answer: Mass fraction of sodium chloride $ \\ omega (NaCl) $ \u003d 7%

Mass fraction of the element in the molecule

The formula of the chemical, for example $ H_2SO_4, carries many important information. It denotes either a separate substance molecule, which is characterized by a relative atomic mass, or 1 mol of a substance that is characterized by a molar mass. The formula shows high-quality (consists of hydrogen, sulfur and oxygen) and quantitative composition (consists of two hydrogen atoms, the sulfur atom and four oxygen atoms). According to the chemical formula, you can find a mass of molecules as a whole (molecular weight), as well as calculate the ratio of masses of elements in the molecule: M (H): M (S): M (O) \u003d 2: 32: 64 \u003d 1: 16: 32. When calculating the ratios of the mass of elements, it is necessary to take into account their atomic mass and the number of corresponding atoms: $ M (H_2) \u003d 1 * 2 \u003d 2 $, $ M (S) \u003d 32 * 1 \u003d 32 $, $ M (O_4) \u003d 16 * 4 \u003d 64 $

The principle of calculating the mass fraction of the element is similar to the principle of calculating the mass fraction of the substance in solution and is located according to the similar formula:

$ \\ Omega_ \\ Text (EL-TA) \u003d \\ DFRAC (AR \\ TEXT (EL-TA)) \\ Cdot N _ (\\ TextRM (atoms))) (M_ \\ Text (Molecules)) (\\ CDOT 100 \\%) $

Find a mass fraction of elements in sulfuric acid.

Decision

Method 1 (proportion):

We find the molar mass of sulfuric acid:

$ M (H_2SO_4) \u003d 1 \\ CDOT 2 + 32 + 16 \\ CDOT 4 \u003d 98 \\ HSPACE (2PT) \\ TextRM (g / mol) $

One sulfuric acid molecule contains one sulfur atom, it means a sulfur mass in sulfuric acid will be: $ M (S) \u003d Ar (S) \\ Cdot n (S) \u003d 32 \\ TextRM (g / mol) \\ CDot 1 $ \u003d 32g / mol

We take the mass of the entire molecule for 100%, and the mass of sulfur - for x% and make up the proportion:

$ M (H_2SO_4) $ \u003d 98 g / mol - 100%

m (s) \u003d 32g / mol - x%

Where $ x \u003d \\ dfrac (32 \\ TextRM (g / mol) \\ CDOT 100 \\%) (98 \\ TEXTRM (g / mol)) \u003d 32, 65 \\% \u003d 32 \\% $

Method 2 (formula):

$ \\ omega (S) \u003d \\ DFRAC (AR _ (\\ Text (EL-TA)) \\ Cdot N _ (\\ TextRM (atoms))) (M_ \\ Text (molecules)) (\\ CDOT 100 \\%) \u003d \\ DFRAC ( Ar (s) \\ CDot 1) (M (H_2SO_4)) (\\ CDOT 100 \\%) \u003d \\ DFRAC (32 \\ TextRM (g / mol) \\ CDot 1) (98 \\ TEXTRM (g / mol)) (\\ CDOT 100 \\%) \\ APPROX32, 7 \\% $

Similarly, by the formula, we calculate the mass fractions of hydrogen and oxygen:

$ \\ omega (H) \u003d \\ DFRAC (AR (H) \\ CDOT 2) (M (H_2SO_4)) (\\ CDOT 100 \\%) \u003d \\ DFRAC (1 \\ TEXTRM (g / mol) \\ CDOT 2) (98 \\ $ \\ omega (O) \u003d \\ DFRAC (AR (O) \\ CDOT 4) (M (H_2SO_4)) (\\ CDOT 100 \\%) \u003d \\ DFRAC (16 \\ TextRM (g / mol) \\ CDOT 4) (98 \\ 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

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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.

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 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

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 to the formula, an exclamation mark that tells us that the task of school (the coarse rounding of atomic masses is used to integer numbers), then the answer is as follows:

The proportion will be like this

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

20.64 tons

Something like this:)

Good luck in the calculations !!

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. In order to obtain a certain amount of a substance in the laboratory, where the exact calculation is very important and all possible outcome of the reaction options, you need to know only a couple of 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 mixture.

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 of 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.

At the moment, about 120 different chemical elements are known, of which in nature can be found no more than 90. The diversity of various chemicals around us is incommensurable more than this number.
This is due to the fact that extremely rare chemicals consist of separate, non-interchangeable atoms of chemical elements. In such a building, only a small number of gases called noble - helium, Neon, Argon, Krypton, Xenon and Radon has such a structure. More often, chemicals are not consisting of disparate atoms, but from their associations in various groups.
That is, atoms of most chemical elements are able to communicate with each other. Most often, as a result, molecules are obtained - particles representing groups of two or more atoms. For example, the hydrogen chemical consists of hydrogen molecules, which are formed from atoms as follows:

Figure 3. Formation of hydrogen molecule

Atoms of different chemical elements can both form connections with each other, for example, the interaction of an oxygen atom with two hydrogen atoms is formed by a water molecule:

Figure 4. Education of water molecule

Since each time you draw atoms of chemical elements and to sign them uncomfortable, chemical formulas were invented to reflect the composition of the molecules. For example, the molecular hydrogen formula is written as H 2, where the number 2 written by the substrate on the right of the symbol of the hydrogen atom means the number of atoms of this type in the molecule. Thus, the water formula can be written as H 2 O. The unit that should show the number of oxygen atoms in the molecule, according to the rules adopted in chemistry, are not written. The numbers denoting the amounts of atoms in the composition of one molecule are called indexes.
Consider some more examples of chemical formulas. Thus, ammonia formula is written as NH 3, which indicates that each ammonia molecule consists of one nitrogen atom and three hydrogen atoms.
We often encounter molecules in which you can count several identical groups of atoms. For example, from Al 2 aluminum sulfate formula (SO 4) 3, it can be concluded that the composition of the molecules of this substance are two groups of SO 4 atoms.
Thus, the chemical formulas of substances are uniquely characterized by both their high-quality and quantitative composition.
Of all the above, the law of constancy of the composition of the substance established in 1808 by the French scientist Joseph Louis Proustom, and he sounds as follows:

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

Since any chemical substance is a combination of the molecules of the same composition, this leads to the fact that the proportions between the atoms of the chemical elements in any portion of the substance are the same as in the same molecule of this substance. All differences in the chemical properties of substances depend on the quantitative and qualitative composition of molecules and in addition, on the order of the bonds of atoms among themselves, if any.
Thus, it is possible to give the following definition of the term molecule:

The molecule is the smallest particle of any chemical possessing it with chemical properties.

Similar to relative atomic mass, there is also such a concept as a relative molecular weight M R.:

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

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

To describe the qualitative and quantitative composition of the substance use such a concept as a mass fraction of the chemical element w (x).