Question

A solution of urea (mol. Mass 56 g mol-1) boils at 100.18oc at the atmospheric pressure. If kf and kb for water 1.86 and 0.512 k kg mol-1 respectively, the above solution will freeze at:

Answer 1

AT; =K;m ..(1)

AT, =Kym

(2)

(3)

AT, —

AT, — depression in freezing point

AT, — elevation in b.p. b.p. of water = 100°C; K,= 1.86 K kg mol!

b.p. of urea in water = 100.18°C

Ky = 0.512 K kg mol

= AT, =0.18

f.p. of water = 0°C

f.p. of urea in water = -7°C

= from cq. (3),

_ 1.86

0.18 0.512

— 7'— 0.6539

= fp. of urea in water = —0.654°C