A compound is known to have a free amino group with a pka of 8.8, and one other ionizable group with pka between 5 and 7. To 100ml of a 0.2M solution...

A compound is known to have a free amino group with a pka of 8.8, and one other ionizable group with pka between 5 and 7. To 100ml of a 0.2M solution of this compound at pH 8.2 was added 40ml of a solution of 0.2M hydrochloric acid. The pH changed to 6.2. The pka of the second ionizable group is?