HA(aq) H+(aq) + A-(aq) The freezing point depression of a 0.1 m aqueous solution of a monobasic weak acid HA is 0.20 C. The dissociation constant for the acid is Given: Kf(H2O) = 1.8 K kg mol-1, molality ≡ molarity
- 1.90 10-3
- 1.38 10-3
- 1.1 10-2
- 1.89 10-1
Answer
(2) 1.38 10-3
Solution
Tf = i Kf m
i = TfKf m = 0.201.8 0.1 = 109 = 1.11
For HA H+ + A-: i = 1 +
= i - 1 = 19 = 0.111
Ka = C 21 - (C = 0.1 M, since molality ≡ molarity)
Ka = 0.1 ( 19 )2 89 = 0.172
Ka = 1.38 10-3
Answer: (2)