For this problem, we are being asked to determine the pH when 150 mL of 0.10 M propionic acid (CH_{3}CH_{2}COOH) is mixed with 400 mL 0.0650 M potassium propionate (CH_{3}CH_{2}COOK)

A solution that contains a weak acid and its conjugate base is a **buffer solution**. The pH of a buffer solution can be determined using the Henderson-Hasselbalch equation.

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

**Given solution:**

A buffer solution is prepared by mixing 150 mL of 0.100 M propionic acid (HC_{3}H_{5}O_{2}, K_{a} = 1.3 x 10^{-5}) and 400 mL of 0.0650 M potassium propionate. What is the pH of the solution? ( hint: keep track of volumes!)

a. 5.13

b. 6.20

c. 4.55

d. 4.31

e. 4.82