Question

If sodium sulphate is considered to be completely dissociated into cations and anions in aqueous solution, the change in freezing point of water ($\Delta T_f$), when 0.01 mol of sodium sulphate is dissolved in 1 kg of water, is ($K_f$ = 1.86 K kg mol⁻¹)
(a) 0.372 K
(b) 0.0558 K
(c) 0.0744 K
(d) 0.0186 K

Answer 1

Answer: 0.1120∘K

Depression of Freezing point,

ΔTf=iKfm, where i is the van't Hoff factor,

Kf is the Freezing Point Depression Constant and m is the molality.

Calculating i for Sodium Sulfate →Na2SO2−4(s)→2Na++SO2−4→i=3

In this case i=3,Kf=1.86 and m=0.02→ΔTf=3×1.86×0.02=0.112