Chemical reactions are typically described as so many moles of compound A reacting with so many moles of compound B to form so many moles of compound C. When we determine how much of a reactant to use, we need to know the number of moles in a given volume of the reactant. Percent solutions only tell us the number of grams, not moles. A 100 mL solution of 2% NaCl will have a very different number of moles than a 2% solution of CsCl. So we need another way to talk about numbers of moles.