An unknown compound X, is thought to have a carboxyl group with a pka of 2.0 and another ionizable group with a pka between 5 and 8. When 75 ml of 0.1 M NaOH was added to 100 ml of a 0.1 m solution of X at pH2.0, the pH increased to 6.72. Calculate the pKa of the second ionizable group of X.
The key is to think of this as a limiting reactant problem.
First, in the intial container of compound X, which is like a diprotic weak acid, H2A or H(sub2)A, we have
pH=pKa1 + …
This solution is found by thinking of this as a limiting reactant problem.