[Jordi]

A compound has a pKa of 7.4. To 100 mL of a 1.0 M solution of this compound at pH 8.0 is added 30 mL of 1.0 M HCl. What is the resulting solution pH?

I use M_1 / V_1 = M_2 / V_2 where M_1 = 10^-8.0 so (0.00000001 M) / 0.100 L = M_2 / (0.130 L) => M_2 = 0.000000013 => pH = 7.9. But the answer is 7.4, and I'm not sure how to get this. Perhaps I incorporate the pKa? How?

[Borek]

This is buffer (Henderson-Hasselbalch equation) question.