112g of iron reacted completely with 64g of sulfur. What mass of iron sulfide was made?

There are two common sulfides of iron:

Iron(II)sulfide is FeS

Iron(III)sulfide is Fe2S3

Since we're told that 112 grams of iron reacts completely with 64 grams of sulfur, we can determine which compound formed by the mole ratio of Fe to S. Find the number of moles of each reactant using their molar mases:

112 g Fe x 1mol/55.8 g = 2.01 moles

64 g S x 1 mol/32.1 g = 1.99 moles S

Since the mole ratio is about 1:1, the formula is FeS and the equation is:

`Fe + S -> FeS`

According to the law of consevation of mass, the mass of the product(s) equals the mass of reactant(s) so the mass of FeS that formed is:

112 g + 64 g = 176 g

This method works because there's only one product and we know that both reactants reacted completely. If there's more than one product or there's an excess of one reactant, you could use dimensional analysis to convert from grams of the limiting reactant to moles, then to moles of the product in question, then to grams of the product.