How to Calculate Normality

How to Calculate Concentration in Normality

Calculate normality as gram equivalent weight of solute per liter of solution.
Calculate normality as gram equivalent weight of solute per liter of solution. Arne Pastoor / Getty Images

The normality of a solution is the gram equivalent weight of a solute per liter of solution. It may also be called the equivalent concentration. It is indicated using the symbol N, eq/L, or meq/L (= 0.001 N) for units of concentration. For example, the concentration of a hydrochloric acid solution might be expressed as 0.1 N HCl. A gram equivalent weight or equivalent is a measure of the reactive capacity of a given chemical species (ion, molecule, etc.).

The equivalent value is determined using the molecular weight and valence of the chemical species. Normality is the only concentration unit that is reaction dependent.

Here are examples of how to calculate the normality of a solution.

Normality Example #1

The easiest way to find normality is from molarity. All you need to know are how many mole of ions dissociate. For example, a 1 M sulfuric acid (H2SO4) is 2 N for acid-base reactions because each mole of sulfuric acid provides 2 moles of H+ ions.

1 M sulfuric acid is 1 N for sulfate precipitation since 1 mole of sulfuric acid provides 1 mole of sulfate ions.

Normality Example #2

36.5 grams of hydrochloric acid (HCl) is a 1 N (one normal) solution of HCl.

A normal is one gram equivalent of a solute per liter of solution. Since hydrochloric acid is a strong acid that dissociates completely in water, a 1 N solution of HCl would also be 1 N for H+ or Cl- ions for acid-base reactions.

Normality Example #3

Find the normality of 0.321 g sodium carbonate in a 250 mL solution.

To solve this problem, you need to know the formula for sodium carbonate. Once you realize there are two sodium ions per carbonate ion, the problem is simple:

N = 0.321 g Na2CO3 x (1 mol/105.99 g) x (2 eq/1 mol)
N = 0.1886 eq/0.2500 L
N = 0.0755 N

Normality Example #4

Find the percent acid (eq wt 173.8) if 20.07 mL of 0.1100 N base is required to neutralize 0.721 g of a sample.

This is essentially a matter of being able to cancel out units to obtain the final result. Remember, if given a value in milliliters (mL), it's necessary to convert it to liters (L). The only "tricky" concept is realizing the acid and base equivalence factors will be in a 1:1 ratio.

20.07 mL x (1 L/1000 mL) x (0.1100 eq base/1 L) x (1 eq acid/1 eq base) x (173.8 g/1 eq) = 0.3837 g acid

When to Use Normality

There are specific circumstances when it's preferable to use normality rather than molarity or other unit of concentration of a chemical solution.

  • Normality is used in acid-base chemistry to describe the concentration of hydronium (H3O+) and hydroxide (OH-). In this situation, 1/feq is an integer.
  • The equivalence factor or normality is used in precipitation reactions to indicate the number of ions that will precipitate. Here, 1/feq is once again and integer value.
  • In redox reactions, the equivalence factor indicates how many electrons can be donated or accepted by an oxidizing or reducing agent. For redox reactions, 1/feq may be a fraction.

Considerations Using Normality

Normality is not an appropriate unit of concentration in all situations.

First, it requires a defined equivalence factor. Second, the normality is not a set value for a chemical solution. Its value can change according to the chemical reaction being examined. For example, a solution of CaCl2 that is 2 N with respect to the chloride (Cl-) ion would only be 1 N with respect to the magnesium (Mg2+) ion.