What Is the Difference Between Molarity and Normality?

Molarity vs Normality

Molarity and normality express a solution's concentration.
Molarity and normality express a solution's concentration. Nicholas Rigg, Getty Images

Both molarity and normality are measures of concentration. One is a measure of the number of moles per liter of solution and the other changes depending on the solution's 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.

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 solution's role in the reaction determines the solution's normality.

For acid reactions, a 1 M H2SO4 solution will have a 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 important part, 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) 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, it's 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 you're 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 aren't this complicated and you're just examining the number of H+ ions in a solution.