What is the deltaG° for the dissociation of hydrofluoric acid in H2O at 298 K if Ka = 7.2 x 10^-4?

Question

HF(aq) H2O(l) --> H3O (aq) F-(aq)

a. 6.3 kJ/mol

b. 7.8 kJ/mol

c. 18 kJ/mol

d. 29 kJ/mol

e. 177 kJ/mol

The answer is c. c. 18 kJ/mol... but could someone please explain to me how you arrive at that value? It would be immensely appriciated!

Answer 1

For Gibb's free energy, we know that

ΔG° = -RTlnK, where

R= 0.008314 kJ K^(-1)mol^(-1) <-- constant

T = temperature = 298

K = Ka = 7.2E-4

Plugging everything in, we get

ΔG° = -RTlnK

ΔG° = -(0.008314)(298)ln(7.2E-4) = 17.93kJ = ~18kJ

[Answer: see above]