Consider a buffered solution at pH = 4.00 made with HF (Ka = 7.2 x 10^-4) and NaF. Which of the following statements is true?
A) [HF] = [F-]
B) [HF] = [H+]
C) [HF] > [F-]
D) [H+] = [F-]
E) [HF] < [F-]
Solution: The equation for calculating pH of a buffer is called Henderson-Hasselbach equation which is
pH = pKa + log ([A-]/[HA]) where HA is the weak acid and A- is its conjugate base. pKa=-log(Ka)
Let's do some plugging.
4.00 = -log(7.2 x 10^-4) + log ([F-]/[HF])
log ([F-]/[HF]) = .857
([F-]/[HF] = 7.2
[F-] is 7.2 times higher than [HF] Choice E