This is a question that had me puzzled for quite a while. I feel that information is missing.
A weak acid, $\ce{HA}$, $K_a = 1.0$ x $10^{-4}$, is titrated with $\ce{NaOH}$. At the equivalence point, the concentration of $\ce{NaA}$ at the equivalence point is $0.010$ molar. What is the pH at the equivalence point?
a) 4.0
b) 6.0
c) 7.0
d) 8.0
e) 11.0
The correct answer is d, which is fairly easy to guess, as the pH needs to be greater than 7, but 11 is much to basic. However, I'm not sure how I would be able to figure that out from the information given? I've tried using the Henderson-Hasselbalch equation, but I can't figure out the relationship between $\ce{[NaA]}$ or $\ce{[A-]}$ and $\ce{[HA]}$.
Any help would be greatly appreciated.