To calculate the molar mass (or effectively, the molecular weight) of a molecule, you must first write out its formula, and
then look up the molecular weights of each atom.
For Example, in water:
There are 2 Hydrogen atoms and 1 oxygen atom.
To find the molar mass of water, take the molar mass of hydrogen and multiply it by the number of atoms of hydrogen in the
compound. Then, add that to the molar mass of oxygen multiplied by the number of atoms of oxygen in the compound.
(1 g/mol of hydrogen)(2) = 2g/mol
+(16 g/mol of oxygen)(1) = 16 g/mol
= 18 g/mol of water
The total molar mass of water is 18 g/mol