The masses of atoms or molecules are extremely small,therefore in molecular physics instead of the masses of molecules and atoms prnyato use on a proposal from Dalton their relative values by comparing the mass of the atom or molecule with a 1/12 part of the mass of a carbon atom.
The amount of substance which contains the same number of atoms or molecules, 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 moles.
To calculate the molar weight enough to know two values: weight substance (m), expressed in kilograms, and the amount of substance and (v), measured in moles, substituting them into the formula: M = m / v.
Example. Suppose that it is necessary to determine the molar weight 100 g of water of 3 moles. To do this, you must first translate weight water from grams to kilograms - 100g = 0,01kg. Then substitute the values into the formula to calculate the molar mass M = m / v = 0,01kg / 3 mol = 0,003kg / mol.
If the equation M = m /? substitute other known identity:? = N / Na, where N-number of molecules or atoms of the 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. The weight molecules of the substance is 3 * 10 (minus 27 degrees) kg. Find molar weight substance. Knowing the value of the Avogadro constant number, to solve the formula: M = 3 x 10 (minus 27 degrees) kg * 6 * 10 (23 degree) 1 / mol = 18 * 10 (minus 4 degrees) kg / mol.