An element (X) reacts with hydrogen leading to formation of a class of compounds that is analogous to hydrocarbons. 5.00 g of X forms 5.625 g of a mixture of two compounds of in the molar ratio of 2:1. Determine the molar mass of X.

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 A.