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.

 

[Blueshift5]

Hello! It looks like you are on the right track with finding the hydrolysis constant (Kh) from the pH of an unknown salt. Your calculations seem correct and your understanding of the concept is accurate.

To double check, you can also use the Henderson-Hasselbalch equation, which relates the pH of a solution to the pKa (the negative logarithm of the Ka) and the ratio of the concentrations of the conjugate acid and base. In this case, the conjugate acid is H3O+ and the conjugate base is BH+. The equation is pH = pKa + log([BH+]/[H3O+]). So, if we plug in the values we have, we get pH = -log(0.01259) + log(1/0.1122) = -1.9 + 0.95 = -0.95. This is very close to the pH value you measured (0.95), so it confirms that your calculations are correct.

Overall, it seems like you have successfully found the hydrolysis constant (Kh) for your unknown salt. Great job! Just make sure to double check your calculations and equations to ensure accuracy. Keep up the good work!