Question

Which is the formula mass of `Na_2SO_4`?

Answer 1

Break up the formula compound and see what are the atoms present.
2 atoms of Na, 1 atom of S, and 4 atoms of Oxygen.

Refer to your periodic table and find out the formula mass for each individual atom present.
Na = 23.0 `xx`2 = 46.0 g/mol
Note: It's multiplied by 2 because there are two atoms of Na.
S = 32.1 `xx` 1 = 32.1 g/mol
O = 16.0 `xx` 4 = 64.0 g/mol

Add the individual formula mass together and therefore it will give you the overall formula mass of `Na_2SO_4`.
46.0 + 32.1 + 64.0 = 142.1 g/mol is the formula mass of `Na_2SO_4`.