A mixture of and is heated until all the water is driven-off. If 5.0 gm of a mixture gives 3g of anhydrous salts, what is the percentage by mass of in the original mixture?

This question uses stoichiometry and limiting reagent. Write the balanced chemical equation, compute moles of each reactant, identify the limiting reagent, and use stoichiometric ratios (and % yield, if needed) to obtain the answer; this gives option C.