Molarity and normality are both units of concentration, but they have different definitions and are used to express concentrations in different ways. The main difference between molarity and normality lies in the way they take into account the chemical reactions that occur in a solution.
Molarity (M) is defined as the number of moles of solute per liter of solution. It is a measure of the concentration of a substance in terms of its molecular weight and volume. Molarity does not consider the stoichiometry of the chemical reaction involved. For example, if you have a solution of hydrochloric acid (HCl), the molarity would simply be the moles of HCl divided by the volume of the solution.
Normality (N), on the other hand, is defined as the number of equivalents of solute per liter of solution. An equivalent is a measure of the reactive capacity of a compound or ion and is based on the number of moles of particles involved in a chemical reaction. Normality takes into account the stoichiometry of the reaction. For instance, if you have a solution of hydrochloric acid (HCl) and you know it reacts with sodium hydroxide (NaOH) in a 1:1 ratio, then the normality of HCl would be equal to its molarity, but the normality of NaOH would be twice its molarity because it supplies two equivalents of hydroxide ions.
In the case of ionic compounds or solutions, where there can be multiple ions present and involved in reactions, normality provides a more accurate representation of the concentration. This is because ionic compounds can dissociate into multiple ions in solution, and the concentration of each ion contributes to the overall reactivity.
To summarize, molarity is a measure of concentration based on moles of solute per liter of solution, while normality takes into account the stoichiometry of the chemical reaction and represents the concentration in terms of equivalents of solute per liter of solution. Normality is particularly useful when dealing with reactions involving multiple ions or compounds.