Help with pI/pH problem? Any input Appreciated!!!

[confused2021]

Hello. My question is as follows :Calculate the pI of Leucine pKa (carboxyl) = 2.4; pKa (amino) = 9.6; What is the ratio of COO- : COOH at pH 3.4?
For pI I get 6 (9.6 + 2.4/2 = 6). I am having great difficulty with the 2nd part of the question. I get 63.8% COO-: 36.2% COOH. I deduced this since 50% is dissociated at pH2.4 and 100% is dissociated at pH6. There are 3.6 units from 2.4 to 6. Since it is one above 2.4, I divided 1/3.4 and multiplied it by 50 to get 13.8%. This added to the 50% dissociated at 2.4pH equals 63.8% dissociated and 36.2% not dissociated.

Any help on how to reason this out to get the right answer would be greatly appreciated!!!!

[BabcockHall]

Did you try using the Henderson-Hasselbalch equation?