Suppose that you have an amino acid with three dissociated groups (a-carboxyl group, a-amino group and basic r-group), with the pkas 3, 9, and 11.
b. what is the pi
c. what is the net charge at ph 6

B. pI=10

C. Charge 1+

Explanation:

B. You'll write your pKa values from low to high, we'll build a table to find the charges for your three posible dissociated groups:

         -COOH   -NH3   -R     Total charge

              0             +        +            2+

pH 3 -----------------------------------------------------

               -             +         +            1+

pH 9------------------------------------------------------

               -             0         +             0

pH 11------------------------------------------------------

               -             0          0             1-

To calculate pI we'll take the pKa values where you have total charge zero (pKa 9 and 11):

Our pI=10

C. considering pH 6 is between pH3 and pH9 we can assume that the net charge in that pH will be 1+.

I hope you find this information useful! Good luck!