Molarity, Molality, or Normality? (A Quick Review)
Updated September 2016
Molarity and Molality
Molarity (M) is defined as the number of moles of solute per liter of solution.
molarity = moles of solute/liters of solution
Molality (m) is defined as the number of moles of solute per kilogram of solvent.
molality = moles of solute/kilograms of solvent
Although their spellings are similar, molarity and molality cannot be interchanged. Molarity is a measurement of the moles in the total volume of the solution, whereas molality is a measurement of the moles in relationship to the mass of the solvent.
When water is the solvent and the concentration of the solution is low, these differences can be negligible (d = 1.00 g/mL). However, when the density of the solvent is significantly different than 1 or the concentration of the solution is high, these changes become much more evident.
Compare the molar and molal volumes of 1 mol of a solute dissolved in CCl4 (d = 1.59/mL).
For a 1 Molar solution, 1 mol of solute is dissolved in CCl4 until the final volume of solution is 1 L.
For a 1 molal solution, 1 mol of solute is dissolved in 1 kg of CCl4.
1 kg of CCl4 × (1,000 g/1 kg) × (mL/1.59 g) = 629 mL CCl4
Normality (N) is defined as the number of mole equivalents per liter of solution:
normality = number of mole equivalents/1 L of solution
Like molarity, normality relates the amount of solute to the total volume of solution; however, normality is specifically used for acids and bases. The mole equivalents of an acid or base are calculated by determining the number of H+ or OH- ions per molecule:
N = n × M (where n is an integer)
For an acid solution, n is the number of H+ ions provided by a formula unit of acid.
A 3 M H2SO4 solution is the same as a 6 N H2SO4 solution.
For a basic solution, n is the number of OH- ions provided by a formula unit of base.
A 1 M Ca(OH)2 solution is the same as a 2 N Ca(OH)2 solution.
Note: The normality of a solution is NEVER less than its molarity!