The molecular mass, molar mass and formula mass of compounds

The molecular mass:

The molecular mass (sometimes called molecular weight) is the sum of the atomic masses (in amu) in the molecule. 

We can calculate the mass of a molecule from the atomic masses of the component atoms. For example, the molecular mass of H₂O is:

2(atomic mass of H) + atomic mass of O

2(1.008amu) + 16.00amu = 18.02amu

If we multiplied the atomic mass of each element by the number of atoms of that element present in the molecule and sum over all the elements we get the molecular mass.

The molar mass of a compound (in grams) is numerically equal to its molecular mass (in amu). Example, the molecular mass of water is 18.02 amu, so its molar mass is 18.02 g. One mole of water weighs 18.02 g and contains 6.022x10²³ H₂O molecules.

Formula mass is used to express mass of ionic compounds such as NaCl and MgO. For example the formula mass of NaCl is sum of the atomic masses of Na⁺ and Cl^- ions.

Thus, the formula mass of NaCl is the mass of one formula unit:

Formula mass of NaCl= 22.99amu + 35.45amu

                   = 58.44amu