[brasarehot]

You titrate 1 L of 1 M PIPES by 10 N NaOH. Calculate the pH of the solution when 2ml of NaOH is added. ANSWER=5.06

Here's my work...but I got the wrong answer. What did I do wrong?
1Mx1L=1mols PIPES
10Mx(2/1000 L) = 0.02mols NaOH
(1-0.02)/(1L + 2/1000 L) = 0.978043912M remaining acid
pKa = [H+]^2 / [HA], 10^-6.76 = [H]^2/(0.978043912), [H] = 0.000412268
-log(0.000412268) = 3.384820823

So I'm getting pH=3.38 instead of 5.06....so what did I screw up? Thanks in advance