How Do You Calculate the Hydrolysis Constant from pH?

[odonneaj]

Hi! First post I must say ... but more importantly I need help finding the hydrolysis constant (Kh) from pH of an unknown salt. This is what i have so far.

We took the measurement of pH of a 1.0 M solution onf an unknown salt in which one of the ions hydrolyzes. The pH our group got was 0.95. This means that Ka > Kb (that's why it's acidic) and this means the cation hydrolyzes.

We know that Kh = Ka = { [H3O+] / [BH+] }

So to find the [H3O+] i did the following steps:

pH = -log[H3O+]
[H3O+] = 10^-pH
[H3O+] = 10^-.95
[H3O+] = .1122

Next we know Ka = { [H3O+][A-] / [HA] } or better known as concentrations of products of concentrations of reactants. So now i get Ka = (.1122)(.1122) / 1 so Ka =.01259

Which as stated above Kh = Ka so am I right with the hydrolyzes constant?

 

[chemisttree]

You are really close. The [BH+] term in your Ka expression is a little off. Remember that at equilibrium the concentration of [BH+] has been reduced by an equal amount as the [H+]. Set [H+] to x and the Ka expression becomes:

Ka = x^2/[BH+ - x]

Substitute a value (that you have) for 'x' and solve from there.