Question

A buffer solution is prepared by mixing 10 mL of 1.0 M acetic acid and 20 mL of 0.5 M sodium acetate and then diluted to 100 mL with distilled water. If the $ p{{K}_{a}} $ of acetic acid is 4.76, what is the pH of the buffer solution prepared?

• A. 3.84
• B. 4.76
• C. 4.34
• D. 5.21

Answer 1

Answer: (B)

Buffer solution is prepared by mixing 10 mL of $ \text{1}\,\text{MC}{{\text{H}}_{\text{3}}}\text{COOH} $ and 20 mL of $ \text{0}\text{.5}\,\text{MC}{{\text{H}}_{\text{3}}}\text{COONa} $ Volume of solution =100 mL Now, the concentration of $ \text{C}{{\text{H}}_{\text{3}}}\text{COOH} $ Before dilution =After dilution $ {{M}_{1}}{{V}_{1}}={{M}_{2}}{{V}_{2}} $ $ 1\times 10=\,{{M}_{2}}\times 100 $ $ {{M}_{2}}=\frac{10}{100}=\frac{1}{10} $ Similarly, the concentration of $ C{{H}_{3}}COONa $ Before dilution = After dilution $ {{M}_{1}}{{V}_{1}}={{M}_{2}}{{V}_{2}} $ $ 0.5\times 20={{M}_{2}}\times 100 $ $ {{M}_{2}}=\frac{0.5\times 20}{100}=\frac{1}{100}M $ Then, $ pH=p{{K}_{aa}}+\log \frac{[Salt]}{[Acid]} $
$ =4.76+\log \frac{0.1}{0.1} $ $ pH=4.76 $