When determining the mass of product used or reactant consumed, the number of moles must be determined before comparing two substances because a) moles react. b) atoms have the same mass but moles are different for each substance. c) different substances have different masses due to their atomic make-up. d) masses are the same for different substances.

moles react.

Explanation:

In a chemical reaction equation, we know that chemical species react in moles. It is actually the moles of the individual chemical species that participate in a chemical reaction and not the masses of those species.

In solving stoichiometric problems, it is mandatory to use moles as a conversion factor in calculating the mass of product formed or the mass of reactant consumed. We often read off the number of moles that reacted from the balanced reaction equation and then convert the reacting mass of species given to the number of moles of that specie that actually participated in the reaction in order to obtain any required information from a reaction.