How can you determine formula mass?
The formula mass is the sum of the atomic masses of the elements making up the compound. For example, sulphuric acid,
Atomic mass of hydrogen (H) - 1.00794 g/mol
Atomic mass of sulphur (S) - 32.065 g/mol
Atomic mass of oxygen (O) - 15.9994 g/mol
So, the formula mass is:
(2 x 1.00794) + 32.065 + (4 x 15.9994) = 98.079 g/mol
This video discusses two additional examples; silicon dioxide and magnesium hydroxide.
Hope this helps!