What Is the Difference Between Molarity and Normality

Both molarity and normality are measures of concentration. One is a measure of the number of moles per liter of solution, while the other is variable, depending on the solutions role in the reaction. What Is Molarity? Molarity is the most commonly used measure of concentration. It is expressed as the number of moles of solute per liter of solution. For example, a 1 M solution of H2SO4 contains 1 mole of H2SO4 per liter of solution. H2SO4 dissociates into H and SO4- ions in water. For every mole of H2SO4 that dissociates in solution, 2 moles of H and 1 mole of SO4- ions are formed. This is where normality is generally used. What Is Normality? Normality is a measure of concentration that is equal to the gram equivalent weight per liter of solution. Gram equivalent weight is a measure of the reactive capacity of a molecule. The solutions role in the reaction determines the solutions normality. For acid reactions, a 1 M H2SO4 solution will have normality (N) of 2 N because 2 moles of H ions are present per liter of solution. For sulfide precipitation reactions, where the SO4- ion is the most significant factor, the same 1 M H2SO4 solution will have a normality of 1 N. When to Use Molarity and Normality For most purposes, molarity is the preferred unit of concentration. If the temperature of an experiment will change, then a good unit to use is molality. Normality tends to be used most often for titration calculations. Converting From Molarity to Normality You can convert from molarity (M)Â  to normality (N) using the following equation: N M*n where n is the number of equivalents Note that for some chemical species, N and M are the same (n is 1). The conversion only matters when ionization changes the number of equivalents. How Normality Can Change Because normality references concentration with respect to the reactive species, its an ambiguous unit of concentration (unlike molarity). An example of how this can work may be seen with iron(III) thiosulfate, Fe2(S2O3)3. The normality depends on which part of the redox reaction youre examining. If the reactive species is Fe, then a 1.0 M solution would be 2.0 N (two iron atoms). However, if the reactive species is S2O3, then a 1.0 M solution would be 3.0 N (three moles of thiosulfate ions per each mole of iron thiosulfate). (Usually, the reactions arent this complicated and youd just be examining the number of H ions in a solution.)