Question

You are given a 0.25 M SO₃^2- solution some of which you place in a flask and a 0.25 M solution of the strong acid hydrochloric acid (HCl) which you place in a burette. You also place a pH probe in the flask to record the pH values. You set about titrating the SO₃^2- with HCl.
If at the middle of a buffer region the pH is 7.25, what is a K_b value for SO₃^2-?

Answer 1

In the middle of the buffer region the concentration of the weak acid ann its conjugate base is the same so:

$pH=pKa+log\frac{[A^-]}{[HA]}$

The second part of the equation is 0, and pKa=pH

Now, you want to calculate the Kb, and you now that pKa+pKb=14

So: pKb=14-pKa=14-7.25=6.75

and $K_b=10^{-6.75}=1.78\times 10^{-7}$