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In an experiment equal weighs of iron and sulphur (1.00g) are heated together and react to form FeS. What fraction of original weight of sulphur is left unreacted?

From Mole concept, we know that in a chemical reaction same number of equivalents of each reactant take part in a reaction and number of equivalent is given by the ratio of weight of reactant by its equivalent weight. Also equivalent weight is defined as the ratio of atomic weight to its valency.

Now, number of equivalents of sulphur available in 1 gram of it = 0.0625 as the equivalent weight of sulphur is 16 and its atomic weight is 32 and valency is 2.

Similarly we have number of equivalents of Fe present in 1 gram of it = 0.0315 as the equivalent weight of iron in 31.75 as its atomic weight is 63.5 and valence is 2

Hence in the reaction of iron and sulphur to form FeS only 0.03125 equivalents of sulphur will be consumed and rest will be left unreacted i.e; almost half ( 50% ) of the original weight of sulphur will be left unreacted.