An uncharacterized compound Z is thought to have an amino group with a pKa of 10.0 and another ionizable group with a pKa between 4 and 7. When 20 mL of .25 M HCl is added to 50 mL of .1 M solution of Z at pH 10.0, the pH decreases to 5.2. Calculate the pKa of the second ionizable group of Z?