Question

Equal mass of iron and sulphur together are heated to form FeS. What fraction of orginal mass is left unreacted ? ( Fe = 56 , S = 32)
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Answer 1

In this reaction, the sulfur melts and reacts with the iron exothermically to form iron(II) sulfide, which contains equal amounts of iron and sulfur. So let their equal mass = w

Then, Mole of Fe = w/56 [given]

And, Mole of sulphur = w/32 [given]

Now it should form mole of FeS = w/88 = .0113 w

Now w/56 = 0.0178 w

The Left = 0.0178 - 0.0113 = 0.0065

Now reacted = 0.0065 * 88/100 = .572

Hence unreacted = 1 - 0.572 = 0.428 = 0.43