Molar Excess and Limiting Quantities

Consider the above example of magnesium carbonate reacting with sulfuric acid  

Any chemical completely used up in the reaction is the limiting reagent. The chemical which is left over at the end is the excess reagent (usually this is the cheapest or most common chemical ). 

It is important to know if one of the reactants is in excess before calculating a percentage yield. 

Let us take an example. Treatment of iron(II) sulfide with hydrochloric acid is a way of generating hydrogen sulfide. (RAMs Fe, 55.8 S, 32.1 Cl, 35.5 H, 1, respectively.) 

If we start with 50 g of iron sulfide and 50 g of hydrogen chloride (in hydrochloric acid), which reagent is in excess Remember, in this reaction we require 2 moles of hydrochloric acid for each mole of FeS. 

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