Molarity vs Molality
Quick Answer
Molarity is moles of solute per litre of solution. Molality is moles of solute per kilogram of solvent.
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What Each One Really Means
Let me tell you something about this one. I have taught solution chemistry for years, and I cannot count how many students have lost marks because of one small letter. Molarity uses volume (litres), molality uses mass (kilograms). That is the whole difference, but it changes everything. You see, volume changes when temperature changes, because liquids expand when hot and contract when cold. Mass does not change, whether you are in Lagos or in London. So if the question mentions a change in temperature, molarity is automatically suspect. This is exactly why colligative property calculations use molality. Freezing point depression, boiling point elevation, osmotic pressure — all of them depend on the number of particles, not on a volume that shifts with the weather. Many students ask me why they got the wrong answer when they used molarity for a boiling point question. The answer is simple. You cannot use a temperature-dependent quantity in a calculation about temperature. Also watch the units. Molarity is capital M, molality is small m. In exams, this distinction is not decoration, it is the answer. Take note of this well.
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Side-by-Side Comparison
| Aspect | Molarity | Molality |
|---|---|---|
| Formula | moles of solute ÷ litres of solution | moles of solute ÷ kilograms of solvent |
| What it uses | Volume of the whole solution | Mass of the solvent only |
| Temperature dependent? | Yes, because volume changes | No, mass stays the same |
| Unit symbol | M (capital M) | m (small m) |
| Mainly used for | General concentration, titration | Colligative properties |
Memory Device
Molarity has 'o' like 'volume' has 'o'. Molality has 'a' like 'mass' has 'a'. Simple.