Solutions

The solution from the following with highest depression in freezing point/lowest freezing point is

1. 180 g of acetic acid dissolved in benzene
2. 180 g of acetic acid dissolved in water
3. 180 g of benzoic acid dissolved in benzene
4. 180 g of glucose dissolved in water

Answer

(2) 180 g of acetic acid dissolved in water

Solution

Tf = i Kf m (1 kg solvent; Kf: water 1.86, benzene 5.12 K kg mol-1; f.p.: water 0 °C, benzene 5.5 °C)

∴ the lowest f.p. needs the largest i × m

(A): m = 18060 = 3; dimer in benzene, i ≈ 0.5; f.p. ≈ 5.5 − 0.5 × 5.12 × 3 ≈ −2.2 °C

(B): m = 3; no dimer in water, i ≥ 1 (i × m ≥ 3); f.p. ≤ −1.86 × 3 = −5.58 °C

(C): m = 180122 = 1.48; dimer, i ≈ 0.5; f.p. ≈ 5.5 − 3.8 ≈ +1.7 °C

(D): m = 180180 = 1; i = 1; f.p. = −1.86 °C

∴ (B) has the most solute particles (largest i × m) and the lowest freezing point

Answer: (2)