Answer:
138.39 g/mol
Step-by-step explanation:
Formula mass is a synonym for molar mass, which is the sum of all the masses of the elements. These can be found on a periodic table.
Because this is hydrated, the formula needs to include H2O.
MgSO4 + H2O
Mg + S + 4O + 2H + O
24.31 + 32.07 + (4*16) + (2*1.008) + 16 = about 138.39