Question #4824d

Start by writing the balanced chemical equation that describes this synthesis reaction

#color(blue)(8)"Fe"_ ((s)) + "S"_ (8(s)) -> 8"FeS"_ ((s))#

As you can see, you have a #color(blue)(8):1# mole ratio between iron and sulfur, which means that regardless of how many moles of sulfur take part in the reaction, the reaction will always consume #color(blue)(8)# times more moles of iron.

In your case, you are told that #2.1# moles of iron must take part in the reaction. You can use the aforementioned mole ratio to find the number of moles of sulfur needed to ensure that all the moles of iron react

#2.1 color(red)(cancel(color(black)("moles Fe"))) * "1 mole S"_8/(color(blue)(8)color(red)(cancel(color(black)("moles Fe")))) = "0.2625 moles S"_8#

To convert this to moles of sulfur, use the molar mass of #"S"_8#

#0.2625 color(red)(cancel(color(black)("moles S"_8))) * "256.52 g"/(1color(red)(cancel(color(black)("mole S"_8)))) = color(green)(bar(ul(|color(white)(a/a)color(black)("67 g")color(white)(a/a)|)))#

The answer is rounded to two sig figs, the number of sig figs you have for the number of moles of iron.