Question

The molal freezing point constant for water is 1.86ºC.
Therefore, the change in freezing point of 0.1 molal of NaCl
solution in water is expected to be
(a) 0.372ºC (b) –1.86ºC (c) –0.186ºC (d) –0.372ºC

Answer 1

answer : option (d) -0.372 °C

given, molal freezing point constant for water , k_f = 1.86 °C/mol

molality of NaCl solution in water, m = 0.1 molal

dissociation reaction of NaCl is ..

NaCl ⇔Na^+ + Cl^-

so, i = 1 - α + α + α = 1 + α

if 100% dissociation happens it means α = 1.

so, i = 2

change in freezing point, ∆T_f = -i × K_f × m

= - 2 × 1.86 × 0.1

= -0.372°C

hence option (d) is correct choice.

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