Why moles are needed to work out concentration?

Answer 1

They are not always needed. A thing called parts per million (ppm or mg/L) exists in analytical chemistry, and compounds are often tested in this way. However, more often is the case that moles per litre is used. The reason for this is that if you are carrying out a simple acid base reaction and they react one molecule of base to every one molecule of acid, you can be way off on weight. Weight is dependent on the weight of each atom, and how many atoms there are is not quickly identifiable without calculation.

Think of it this way. a mole is a number that represents a certain amount of molecules (a much more managable number).

If we put on a reaction in which the ratio of reactants is 1:1, if we know the moles of each, we can straight away tell which compound will limit how much product can be formed (when the reactant with smaller number of moles is consumed, the excess of the other reactant cannot magically react with what doesn't exist of the limiting reagent). You don't need molarity for this, but it is the same idea.