Difference Between Molarity and Molality – Understand Chemistry Concentration

M or m?

Molarity and molality are both measurements dealing with concentration of solutions in chemistry. When you see a bottle marked with either a capital M or a lower-case m, what does it mean? Is there a difference?

Yes there is a difference.

Molarity is the most common concentration measurement and denoted by the capital letter M. Molarity is the number of moles of something per volume of mixture containing the something. In solutions, it is the number of moles of solute present per liter of solution.

A 1 M (or 1 molar) solution of NaCl contains one mole of sodium chloride for every liter of solution.

Molality is another concentration measurement. It is the number of moles of solute per unit mass of the solvent. In SI units, the unit of molality is mol/kg. Some texts use the unit ‘molal’, but the official unit is mol/kg.

For aqueous solutions (solutions where the solvent is water) around room temperature, the difference between molarity and molality is very slight. One kilogram of water at room temperature is very close to one liter.

The key points to remember:

M – Molarity – moles per volume of solution – unit: mol/L
m – molality – moles per kilogram of solvent – unit: mol/kg