Consider a Buffer Solution Containing a Weak Acid, HX, and a Salt

Multiple Choice

Consider a buffer solution containing a weak acid, HX, and a salt of the weak acid's conjugate base, NaX. Under what conditions does the pH just equal the p K _a of the weak acid?

A) When the p K _a equals 7.0.
B) When [HX] > [NaX].
C) When [HX] < [NaX].
D) When [HX] = [NaX].
E) When there is no NaX present.