The mass of a molecule is obtained by adding the masses of the atoms that compose it. Let's see an example:
Calculate the molecular mass of water knowing that the atomic masses of hydrogen and oxygen are 1,008 and 15,999 amu.
The water molecule contains two hydrogen atoms and one oxygen, its mass will be given by the sum of two times the mass of hydrogen plus the mass of oxygen.
m(H 2 O) = 2m(H 2 )+ m(O 2 ) = 2x1,008 + 15,999 = 18,015 amu
A mole of a substance represents the quantity in grams equal to the molecular weight and contains 6.02214x10 23 molecules.
The molecular mass of water is 18.015 amu, therefore, a mole of water is 18.015 g and contains 6.02214x10 23 water molecules.