Are molarity and molality the same thing?
Short answer: Molality uses kg of solvent (mass), molarity uses litres of solution (volume, which expands with heat).
The trap
The wrong idea
“Molarity and molality are the same thing.”
It sounds reasonable, and a lot of JEE/NEET aspirants carry it into the exam. Here's a one-question check, then the explanation that makes it click.
Quick check
Try it before you peek
Which concentration does NOT change with temperature?
- Molarity
- Molality
- Both change
Show the answer
B. Molality
Molality uses kg of solvent (mass), molarity uses litres of solution (volume, which expands with heat).
The click
They sound alike but measure different things. Molarity (M) = moles of solute ÷ volume of solution in litres, in mol/L. Molality (m) = moles of solute ÷ mass of solvent in kilograms, in mol/kg. There are two differences: volume versus mass, and solution versus solvent.
Volume depends on temperature: most liquids expand when heated. The moles of solute don't change, but the volume of the solution grows, so the molarity falls slightly. Mass does not depend on temperature, so the molality stays exactly the same. That is why molality is used for colligative properties such as boiling-point elevation (ΔT_b = K_b·m) and freezing-point depression, where temperature changes are the whole point.
The numbers usually differ too. Dissolve 1 mol of solute in 1 kg of water and the molality is exactly 1 mol/kg, but the solution's volume is generally not exactly 1 L, so the molarity is not exactly 1 M. For very dilute aqueous solutions near room temperature the two are numerically close, because 1 L of water has a mass of about 1 kg, and that closeness is what tempts people to treat them as the same.
Mole fraction and mass percentage are also independent of temperature, for the same reason: neither involves a volume.
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