A 1.0 liter solution contains 0.25 M hydrogen fluoride (HF)and 0.38 M sodium fluoride (NaF).What is the pH of this solution? (K_a for HF is 7.2 × 10^-4) A) 3.14 B) 3.32 C) 2.96 D) 0.18 E) 10.68

Question

Quizplus · Accepted Answer

HF is a weak acid and NaF is its conjugate base, which makes this a buffer solution. When calculating the pH of a buffer solution, we use the Henderson-Hasselbalch equation:

pH = pKa + log([A-]/[HA])

where pKa is the dissociation constant of the weak acid (HF), [A-] is the concentration of the conjugate base (NaF), and [HA] is the concentration of the weak acid (HF).

The pKa of HF is given as 7.2 × 10-4. 
Plugging in the values: 
pH = -log(7.2 × 10-4) + log(0.38/0.25) = 3.32

Therefore, the best choice is B.

A 10 Liter Solution Contains 0