Tip 1: How to calculate the molar mass of the substance

Molar mass substance - is the mass of one mole, that is such an amount thereof, which is 6.022 * 10 ^ 23 elementary particles - atoms, ions or molecules.Unit of measure it - grams / mole.
To calculate the molar mass , you need only periodic table, a basic knowledge of chemistry and the ability to perform calculations, of course.For example, a widely known substance - sulfuric acid.It is so widely used in various industries that rightly bears the name "blood chemistry."What is its molecular weight?
Write the exact formula of sulfuric acid: H2SO4.And now take the periodic table and see what the atomic weights of all the elements included in its composition.These three elements - hydrogen, sulfur and oxygen.The atomic mass of hydrogen is 1, the sulfur - 32, oxygen - 16. Therefore, the total molecular weight of the sulfuric acid, based on the index, equal to: 1 * 2 + 32 + 16 * 4 = 98 amu (atomic mass units).
Now let's recall another definition of mole: is the amount of
substance , whose mass in grams is numerically equal to its mass expressed in atomic units.Thus, it turns out that 1 mole of sulfuric acid weighs 98 grams.Here is its molar mass.The problem is solved.
Suppose you set the following conditions: there are 800 milliliters of 0.2 molar solution (0.2 M), some salt, and it is known that in dry form the salt weighs 25 grams.It is required to calculate the molar mass .
To begin remember the definition of a 1 molar (1M) solution.This solution, in 1 liter of which contains 1 mole of any substance .Accordingly, in 1 liter of 0.2M solution would contain 0.2 mol substance.But you do not have 1 liter and 0.8 liters.Consequently, the fact you have a 0.8 * 0.2 = 0.16 mol substance .
And then everything becomes simple.If 25 grams of salt on the conditions of the problem is 0.16 mole, which is equal to the amount of one mole?Making the calculation in one step, you will find: 25 / 0.16 = 156.25 grams.The molar mass of salt is 156.25 g / mol.The problem is solved.
you used in the calculation of the gross values ​​of the atomic weights of hydrogen, sulfur and oxygen.If you want to make calculations with great accuracy, rounding unacceptable.

Tip 2: How to calculate the molar mass

masses of atoms or molecules are extremely small, so in molecular physics instead of the masses of molecules and atoms prnyato use on a proposal Dalton their relative values, comparing mass molecule oratom with 1/12 part of the mass of an atom of carbon.The amount of substance which contains the same number of molecules or atoms, as they contain 12 grams of carbon, called mole.The molar mass of a substance (M) - is the mass of one mole.Molar mass - the value of a scalar, it is measured in the international SI system in kilogammah divided by the mole.
How to calculate the molar mass
To calculate the molar mass enough to know two values: mass substance (m), expressed in kilograms, and the number of substances (v), measuredin moles, substituting them into formula: M = m / v.
example.Suppose it is necessary to determine the molar mass 100 g water 3 moles.To do this, you must first translate mass of water in grams to kilograms - 100g = 0,01kg.Then substitute the value into the formula to calculate the molar mass M = m / v = 0,01kg / 3mol = 0,003kg / mol.
If the equation is M = m /?substitute other known identity:? = N / Na, where N-number of molecules or atoms substance Na- Avogadro constant equal to 6 * 10 to 23 degrees, the molar mass is calculated by another formula: M = m0 * Na.That is, there is another formula for calculating the molar mass.Example 2.
molecule weight substance is equal to 3 x 10 (minus 27 degrees) kg.Find the molar mass substance.Knowing the value of a constant number of Avogadro, to solve the formula: M = 3 x 10 (minus 27 degrees) kg * 6 * 10 (23 degrees) 1 / mol = 18 * 10 (minus 4 degrees) kg / mol.
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