Computing Formula Mass

Element	Atoms in Formula (NH4)2SO4	Atomic Mass	Mass in Formula
N H S O	2 8 1 4	14.01 amu  1.008 amu 32.07 amu 16.00 amu	28.02 amu   8.064 amu  32.07 amu  64.00 amu
		Total Formula Mass	132.154 amu 132.15   amu rounded

Since ammonium sulfate exists in a crystal lattice we shouldn't call its formula mass a molecular mass. Its smallest subunit does have a mass of 132.15 amu, though, which leads to the sloppy usage of saying "the molecular weight of ammonium sulfate is 132.15." For a more useful term that avoids all this nit-picking, see molar mass.