Question

Find the pH of each of the following solutions of mixtures of acids.

9.5×10⁻² M in HF and 0.230 M in HC6H5O

Answer 1

Ka of HF = 6.3 x 10^-4

HF -------------------> H+    + F-

9.5x10^-2                0           0

9.5x10^-2-x             x             x

Ka = x^2 / 9.5 x 10^-2 -x

6.3 x 10^-4 = x^2 / 9.5 x 10^-2 -x

x^2 + 6.3 x 10^-4 x - 5.98 x 10^-5 = 0

x = 7.42 x 10^-3

[H+] = 7.42 x 10^-3 M ------------------------->1

HC6H5O , Ka = 1.3 x 10^-10

[H+] = sqrt (Ka x C) = sqrt (1.3 x 10^-10   x 0.230) = 5.47 x 10^-6 M

totatl [H+] = 7.42 x 10^-3 M + 5.47 x 10^-6 M

                 = 7.42 x 10^-3 M

pH = -log [H+] = -log ( 7.42 x 10^-3 M)

pH = 2.13