The fe2+(aq) solution described in part a was produced by dissolving 4.05 g of ore in acid. what was the percentage of fe by mass in this ore sample?

Percentage by mass is the amount in mass of a component in a mixture per 100 unit of mass of the total mixture. Percentage by mass is the same as  %w/w. In order to determine this value, we need to know the mass of the  substance in question and the total mass of the sample. In this case, we have to know the mass of Fe by doing certain calculations. Then, we divide this value by the total mass given of the ore, 4.05 g and multiply by 100%. It should be expressed as follows:

Percent by mass = mass of Fe in ore / 4.05 g ore sample x 100

However, the question is lacking some information so we cannot really specify the answer. 

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.