Normality Definition in Chemistry

What Normality Is and How to Calculate It

Normality
Normality is a unit of concentration of chemical solutions. Steve McAlister / Getty Images

Normality Definition

Normality is a measure of concentration equal to the gram equivalent weight per liter of solution. Gram equivalent weight is the measure of the reactive capacity of a molecule. The solute's role in the reaction determines the solution's normality. Normality is also known as the equivalent concentration of a solution.

Normality Equation

Normality (N) is the molar concentration ci divided by an equivalence factor feq:

N = ci / feq

Another common equation is normality (N) equal to the gram equivalent weight divided by liters of solution:

N = gram equivalent weight / liters of solution (often expressed in g/L)

or it may be the molarity multiplied by the number of equivalents:

N = molarity x equivalents

Units of Normality

The capital letter N is used to indicate concentration in terms of normality. It may also be expressed as eq/L (equivalent per liter) or meq/L (milliequivalent per liter of 0.001 N, typically reserved for medical reporting).

Examples of 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.

Example Problem

Find the normality of 0.1 M H2SO4 (sulfuric acid) for the reaction:

H2SO4 + 2 NaOH → Na2SO4 + 2 H2O

According to the equation, 2 moles of H+ ions (2 equivalents) from sulfuric acid react with sodium hydroxide (NaOH) to form sodium sulfate (Na2SO4) and water. Using the equation:

N = molarity x equivalents
N = 0.1 x 2
N = 0.2 N

Don't be confused by the number of moles of sodium hydroxide and water in the equation.

Since you've been given the molarity of the acid, you don't need the additional information. All you need to figure out are how many moles of hydrogen ions are participating in the reaction. Since sulfuric acid is a strong acid, you know it completely dissociates into its ions.

Potential Issues Using N for Concentration

Although normality is a useful unit of concentration, it can't be used for all situations because its value depends on an equivalence factor that can change based on the type of chemical reaction of interest. As an example, a solution of magnesium chloride (MgCl2) may be 1 N for the Mg2+ ion, yet 2 N for the Cl- ion. While N is a good unit to know, it's not used as much as molarity or molality in actual labwork. It has value for acid-base titrations, precipitation reactions, and redox reactions. In acid-base reactions and precipitation reactions, 1/feq is an integer value. in redox reactions, 1/feq may be a fraction.