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Consider a buffered solution at pH = 4.00 made with HF (Ka = 7.2 x 10-4) and NaF. Which of the follo

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


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