How to Calculate Molar Mass
The molar mass is the mass of one mole of a sample. To find the molar mass, add the atomic masses (atomic weights) of all of the atoms in the molecule. Find the atomic mass for each element by using the mass given in the Periodic Table or table of atomic weights. Multiply the subscript (number of atoms) times the atomic mass of that element and add the masses of all of the elements in the molecule to get the molecular mass. Molar mass usually is expressed in grams (g) or kilograms (kg).
Molar Mass of an Element
The molar mass of sodium metal is the mass of one mole of Na. You can look up that answer from the table: 22.99 g. You may be wondering why the molar mass of sodium isn't just twice its atomic number, the sum of the protons and neutrons in the atom, which would be 22. This is because the atomic weights given in the periodic table are an average of the weights of the isotopes of an element. Basically, the number of protons and neutrons in an element may not be the same.
The molar mass of oxygen is the mass of one mole of oxygen. Oxygen forms a divalent molecule, so this is the mass of one mole of O2. When you look up the atomic weight of oxygen, you find it is 16.00 g. Therefore, the molar mass of oxygen is:
2 x 16.00 g = 32.00 g
Molar Mass of a Molecule
Apply the same principles to calculate the molar mass of a molecule. The molar mass of water is the mass of one mole of H2O. Add together the atomic masses of all of the atoms of hydrogen and water in a molecule of water:
2 x 1.008 g (hydrogen) + 1 x 16.00 g (oxygen) = 18.02 g