We have to calculate the pH of a buffer that is 0.6 M HF and 0.2 M NaF.

**HF is a weak acid** which **dissociates partially** in an aqueous solution to produce H_{3}O^{+} and F^{-} ions.

HF_{(aq)} + H_{2}O_{(l)}⇌ F^{−}_{(aq)} + H_{3}O^{+}_{(aq)}

The fluoride ion (F^{-}) produced as a result of HF dissociation is the conjugate base of HF.

We will use the **Henderson-Hasselbalch equation** to solve this problem.

$\overline{){\mathbf{pH}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}{{\mathbf{pK}}}_{{\mathbf{a}}}{\mathbf{}}{\mathbf{+}}{\mathbf{}}{\mathbf{log}}\frac{\left[\mathbf{conjugate}\mathbf{}\mathbf{base}\right]}{\left[\mathbf{weak}\mathbf{}\mathbf{acid}\right]}}$

What is the pH of a buffer that is 0.6 M HF and 0.2 M NaF? The Ka of HF is 6.8 × 10^{−4}.

a. 3.65

b. 2.69

c. 3.17

d. 3.50