The depressions in freezing point of 0.01 M aqueous solutions of urea, sodium chloride and sodium sulphate (assuming complete dissociation of the salts) are in the ratio:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 1:2:3
Depression of freezing point is a colligative property: it depends on the number of solute particles. For an electrolyte the van’t Hoff factor i counts the particles each formula unit gives:
\( \displaystyle \Delta T_f=i\,K_f\,m \)
All three solutions have the same concentration and the same solvent, so ΔTf is in the ratio of their i values.
Urea, \( \ce{NH2CONH2} \) — a non-electrolyte; it does not dissociate, so i = 1.
Sodium chloride — \( \ce{NaCl -> Na^+ + Cl^-} \), so i = 2.
Sodium sulphate — \( \ce{Na2SO4 -> 2Na^+ + SO4^2-} \), so i = 3.
The depressions are in the ratio 1 : 2 : 3.
NCERT (2026-27 edition):
1.7 Abnormal Molar Masses – “… In case of association, value of i is less than unity while for dissociation it is greater than unity. For example, the value of i for aqueous KCl solution is close to 2, while the value for ethanoic acid in benzene is nearly 0.5.”
“Inclusion of van’t Hoff factor modifies the equations for colligative properties as follows: … Depression of Freezing point, ΔTf = i Kf m …”