Question

calculate the pH of a 4.20x10^-8 M solution of HBr with water

Answer 1

Given the concentration of HBr is = 4.20x10^-8 M

HBr H⁺ + Br^-

1 mole of HBr produces 1 mole of H⁺

So [H⁺] = [HBr] = 4.20x10^-8 M

As the concentration of HBr is very low the concentration of H⁺ from water also taken into consideration.

The ionic product of water is , k_w = [H⁺] [OH^-] = 1.0x10^-14 M2

In water [H⁺] = [OH^-] , so [H⁺] = 1.0x10^-7 M

So the total concentration of H⁺ = [H⁺] _HBr + [H⁺] _water

                                               = (4.20x10^-8 M) + (1.0x10^-7 M )

                                               = 1.42x10^-7 M

So pH = - log [H⁺]_total

          = - log(1.42x10^-7 M)

          = 6.85

Therefotre the pH of the solution is 6.85