Every mole of any compound has a fixed mass in grams and a fixed number of molecules, namely 6.02x10^23. What we need to do here is:
find out how many moles of the compound we have
find out how many grams in a mole of the compound
multiply moles times grams per mole.
Each molecule of Fe2(SO4)3 has 3 sulfate ions SO4. So a mole of iron sulfate would contain 3 x 6.02x10^23=18.06 x 10^23 sulfate ions. We have 3.59 x 10^23 sulfate ions so the number of moles we have is 3.59/18.06 = 0.19878 moles of iron sulfate.
each mole of iron sulfate has an atomic weight which is the sum of the stomic weights of all the atoms that make up a molecule.
at.weight = 2Fe + 3S +3x4 O= 2 (55.847) + 3 (32.06) + 12 (15.9994) = 399.8668. The mass in grams of one mole of iron sulfate is 399.8668 g
thhe mass of .19878 moles = .19878 X 399.8668g = 79.486 g