Calculate the formula mass for the compound tin(iv) sulfate.

The molecular formula for tin(IV)  sulfate is Sn(SO₄)₂.
mass of sulfur S = 32.065
as there is 2 sulfur in tin (IV) sulfate = 32.065 x 2 = 64.13
mass of oxygen = 15.999 and there are 8 oxygen atoms in the formula; 
8 x 15.999 = 127.992
mass of tin = 118.71
now add all the masses = 64.13 + 127.992 + 118.71 = 310.832 
formula mass of tin(IV) sulfate is 310.832

Answer two atoms of oxygen;