Question

Calculate the pH of a mixture that contains 0.18 M of HCl and 0.25 M of HBrO.

Answer 1

Although HCl is an stron acid but HBrO is not . Ka of HBrO is 2*10^-9 M

0.18 M HCl gives 0.18 H+. Now lets find net concentration of H+ whe HBrO is also there.

                                 HBrO ---->   H+   + BrO-

initial:                      0.25 0.18 0

final:                      0.25-x            0.18+x   x

Ka = [H+][BrO-] / [HBrO]

2*10^-9 = (0.18+x)*x / (0.25-x)

Since Ka is very small, x willl be very small and it can be ignored as compared to 0.25 and 0.18,

so above expression becomes:

2*10^-9 = (0.18)*x / (0.25)

x = 2.77*10^-9 M

[H+] = 0.18 M + 2.77*10^-9 M = 0.18 M

pH = -log [H+]

   = -log (0.18)

= 0.744

Answer: 0.744