The ka of hypochlorous acid (hclo) is 3.0 ⋅ 10-8 at 25.0 °c. calculate the ph of a 0.0375m hypochlorous acid solution.

We can set up an ICE table for the reaction:

HClO H + ClO-

Initial 0.0375 0 0

Change - x + x + x

Equilibrium 0.0375-x x x

We calculate [H+] from Ka:

Ka = 3.0x10^-8 = [H+][ClO-]/[HClO] = (x) (x) / (0.0375-x)

Approximating that x is negligible compared to 0.0375 simplifies the equation to

3.0x10^-8 = (x) (x) / 0.0375

3.0x10^-8 = x2/0.0375

x2 = (3.0x10^-8) (0.0375) = 1.125x10^-9

x = sqrt (1.125x10^-9) = 0.0000335 = 3.35x10^-5 = [H+]

in which 0.0000335 is indeed negligible compared to 0.0375.

We can now calculate pH:

pH = - log [H+] = - log (3.35 x 10^-5) = 4.47