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Everyone who has already moved a little in studying chemistry, faces the concept of mole. True, most immediately think about moths, who ate a fur coat in the closet for the summer, but mole in chemistry is a completely different story . And now we'll figure it out.

Everyone who has already moved a little in studying chemistry, faces the concept of "mole." True, most immediately think about moths, who ate a fur coat in the closet for the summer, but mole in chemistry is a completely different story . And now we'll figure it out.


So, let's look at some chemical reaction. For example, such:

H2 + F2 = 2HF

Here, 1 hydrogen molecule H2 reacts with one fluorine F2 molecule and two hydrogen fluoride molecules is obtained. Let me remind you, the number of molecules or atoms that react or obtained in the reaction is determined by the coefficient, that is, the digit facing the formula of the substance. In our example, there is nothing before hydrogen, but in fact we can deliver a unit here, that is, we need 1 hydrogen molecule. Before Fluoro, it is also worth nothing, it means that we need 1 fluorine molecule. But in front of hydrogen fluoride HF is a twice. This means that we have 2 hydrogen fluoride molecule. I.e:

H2 + F2 = 2HF is the same as

1 H2 + molecule 1 molecule F2 = 2 HF molecules.

But you know that molecules are so small that we cannot see them. How do we consider these molecules that react? To do this, have introduced the concept Mole .

Mol is the amount of substance in which the same particles are contained as atoms are contained in 12 grams of carbon with a atomic unit of mass 12.

This is a rather surrounding definition, but it needs to be remembered. There is a pleasant moment: in one mole of any substance contain Number of Avogadro Particles. Here it is, this is the number:

Formula of the unit of atomic carbon mass

Such a number is difficult. You just think, a billion is 1,000,000,000. And in one mole of particles 6.02 * 100,000,000,000,000,000,000! (But not to see nightmares at night, just remember 6.02 * 10 in twenty-third degree).

So, In one mole of any substance contains 6.02 * 10 in the twenty-third degree of particles. But we know that atoms of different substances have a different structure, and therefore a different mass. That's why The masses of one praying among different substances vary . To sort out this, let's go to the country and do the experiment.

We remember exactly that 1 mol is always the same number of particles (6.02 * 10 in twenty-third degree). But in the usual life of such numbers there is no, so we take the number less, for example, 100. It will be our conditional experimental mole. Now in one pile we put 100 cherries, in another bunch - 100 pears, in the third - 100 watermelons. A bunch is 1 mol. In every pile, we conscientiously folded the same number of particles, right? But particles of these different types: in one pile of Cherry, in the other - pears, in the third - watermelon. And now we will weigh. What do you think there will be a mass of 100 cherries, 100 pears and 100 watermelons? Of course, it will be. At the same time, please note: the number of particles in each pile is equally, but these piles weigh differently. Why? Because the particles are different!

In chemistry everything is the same. If you take 1 mol of hydrogen, 1 moth oxygen and 1 mol sodium, then mass will be different (remember the trip to the country). And it is important. But now there is a lawsager question: what to find out what is the mass of 1 mol of hydrogen, 1 mol of oxygen and 1 mol sodium and in general any substance? For this introduced the concept molar mass.

Molar mass and there is a mass of 1 praying substance.

How to determine it? Just. This is an atomic mass or molecular weight of the substance that we expect, using the Mendeleev table. The molar mass is denoted by the letter M and is expressed in g / mol (just because it shows how many grams lead 1 mol). Examples from chemistry textbook.

Example 1.

Find a lot of one pray (it is molar mass ) Aluminum.

We solve chemistry and look at the Mendeleev table. We see that the atomic mass of aluminum 27. The formula is simply aluminum substance - Al, that is, the atom here is one. Consequently, the molar mass of aluminum coincides with the atomic and equal to 27 g / mol.

Example 2.

Find the molar mass of fluorine.

Fluorine under us under normal conditions - gas, so the fluorine molecule consists of two atoms and looks like this: F2. In the periodic table we find fluorine and see that its atomic mass is 19. Therefore, the molar weight of fluorine 2 * 19 = 38 g / mol.

Example 3.

Find the molar mass of calcium oxide.

Formula Calcium oxide Sao. We look again in the table: Calcium atomic mass 40, atomic mass of oxygen 16. Molar mass of calcium oxide 40 + 16 = 56 g / mol.

Example 4.

Find the molar mass of silicon oxide.

SiO2 silicon oxide formula. The Mendeleev Table reports that the atomic mass of silicon 28, oxygen - 16. Be careful, in this matter of the trick! In the oxide formula, two oxygen atoms, be sure to consider this so that the answer is correct. And it will be like this: the molar mass of silicon oxide 28 + 16 * 2 = 60 g / mol. (16 is the mass of one oxygen atom, we have two in the formula, so we multiplied 16 to 2!).

Example 5.

A complex example of chemistry tutor. But I recommend to penetrate and figure out to clarify everything finally. So, answer, what is the molar mass of sulfuric acid.

Here you have to focus not get confused. The formula of sulfuric acid H2SO4, that is, we have:

· 2 hydrogen atoms

· 1 sulfur atom

· 4 oxygen atoms.

We look into the periodic table and determined atomic masses:

· Atomic weight of hydrogen - 1

· Sulfur atomic weight - 32

· Atomic mass of oxygen - 16.

Go to the calculation:

2 hydrogen atom + 1 sulfur atom + 4 oxygen atom

2 * 1 + 1 * 32 + 4 * 16

In this expression in each term, the first factor is the number of element atoms, the second factor is atomic mass. Then just mathematics: 2 * 1 + 1 * 32 + 4 * 16 = 98. And yes, molar mass Sulfuric acid 98 g / mol.

I am sure, now you will distinguish the mole in the closet and mole in chemistry. And then we will understand, how to weigh on ordinary scales these moths .

Please write in the comments that remained incomprehensible, and I will definitely give additional explanations. Be complain about difficulties in learning the school course and say that you have been scared in the textbook of chemistry. And then the next article will tell exactly about this problem.

Molar mass

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

What is a molar mass?

The founder of the theory of atomic masses was a dalton scientist, which was the table of atomic masses and took the mass of the hydrogen atom per unit.

The molar mass is the mass of one praying substance. Mol, in turn, the amount of substance containing a certain amount of smallest particles that are involved in chemical processes. The number of molecules contained in the same mole is called the Avogadro number. This value is constant and does not change.

Формула числа Авогадро

Fig. 1. The formula of Avogadro.

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

The Avogadro number received its name in honor of the Italian scientist Amedeo Avagadro, who proved that the number of molecules in the same gases is always the same

The molar mass in the international SI system is measured in kg / mol, although usually this magnitude is expressed in grams / mole. This value is denoted by the English letter M, and the formula of the molar mass looks like this:

M = M / V,

where M is the mass of the substance, and V is the amount of substance.

Расчет молярной массы

Fig. 2. Calculation of the molar mass.

How to find a molar mass of substance?

Calculate the molar mass of a substance will help the Table D. I. Mendeleev. Take any substance, for example, sulfuric acid. The formula looks like this: H 2SO. 4. Now we turn to the table and see what the atomic mass of each of the elements included in the acid. Sulfuric acid consists of three elements - hydrogen, sulfur, oxygen. The atomic mass of these elements is respectively 1, 32, 16.

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

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

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

Currently, a unified measurement system has been adopted in physics and chemistry. As standard E.A.M. 1/12 selected part of the mass of carbon atom.

Формула единицы атомной массы углерода

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

The molar mass of any gas or steam is measured very easily. It is enough to use control. The same volume of the gaseous substance is equal to the amount of substance to another at the same temperature. A known method of measuring the volume of the steam is the determination of the amount of displaced air. Such a process is carried out using a side removal leading to the measuring device.

The concept of molar mass is very important for chemistry. Its calculation is necessary to create polymer complexes and many other reactions. In pharmaceuticals, with the help of molar mass, the concentration of this substance in the substance is determined. Also, the molar mass is important in providing biochemical studies (exchanged process in the element).

In our time, due to the development of science, the molecular masses of almost all components of blood, including hemoglobin, are known.

What did we know?

In the 8th grade in chemistry, the "molar mass of the substance" is an important topic. Molar mass is an important physical and chemical concept. The molar mass is the characteristic of the substance, the ratio of the mass of the substance to the number of moles of this substance, that is, the mass of one praying substance. It is measured in kg / mol or gram / mol.

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