On which one of the following factor does the molal depression constant depends on?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Nature of solvent
The depression of freezing point of a dilute solution is proportional to its molality:
\( \displaystyle \Delta T_f=K_f\,m \)
The proportionality constant Kf, the molal depression constant, is the depression for a 1 molal solution. It is worked out from the molar mass, freezing point and enthalpy of fusion of the solvent only, so it depends on the nature of the solvent; for example it is 1.86 K kg mol−1 for water and 5.12 K kg mol−1 for benzene.
Why the other options are incorrect
Option (1) Nature of solute — Incorrect. Freezing point depression is a colligative property; the kind of solute particle does not matter.
Option (2) Amount of solute — Incorrect. The amount of solute changes the molality m and so ΔTf, but not the constant Kf.
Option (4) Amount of solvent — Incorrect. Kf is defined per kg of solvent (1 molal), so the actual amount of solvent does not change it.
NCERT (2026-27 edition):
Depression of Freezing Point – …
