A solution is prepared by mixing 10 mL of 1.0M acetic acid and 20 mL of 0.5M sodium acetate and diluted to 100 mL. If the pK_a of acetic acid is 4.76, then the pH of the solution is

Mole of sodium acetate = 20×0.5=10

Mole of acetic acid=10×1=10
Total volume after dilution = 100+20+10=130 ml

$$\begin{gathered} \left[\mathrm{Sodium} \mathrm{acetate}\right]=\frac{10}{130} \\ \left[\mathrm{acetic} \mathrm{acid}\right]=\frac{10}{130} \end{gathered}$$

Henderson Equation:

$$\begin{gathered} \mathrm{pH}={\mathrm{pK}}_{\mathrm{a}}+\log \left[\frac{\mathrm{Salt}}{\mathrm{acid}}\right] \\ \mathrm{pH}=4.76+\log \left[\frac{{\displaystyle \frac{10}{130}}}{\frac{10}{130}}\right] \\ \mathrm{pH}=4.76 \end{gathered}$$