Question

A buffer is made with sodium acetate (CH3COONa) and acetic acid (CH3COOH); the Ka for acetic acid is 1.80×10−5. The pH of the buffer is 3.98. What is the ratio of the equilibrium concentration of sodium acetate to that of acetic acid if you assume that the Henderson-Hasselbalch equation is accurate?

A. -0.760
B. 0.174
C. 0.840
D. 5.75
E. Not enough information is given to answer this question.

Answer 1

Answer:

B. 0.174

Explanation:

A buffer is formed by a weak acid (CH₃COOH) and its conjugate base (CH₃COO⁻, coming from sodium acetate). We can calculate the pH of this buffer using the Henderson-Hasselbalch equation.

pH = pKa + log [CH₃COO⁻]/[CH₃COOH]

pH - pKa = log [CH₃COO⁻]/[CH₃COOH]

3.98 - (-log 1.80 × 10⁻⁵) = log [CH₃COO⁻]/[CH₃COOH]

-0.760 = log [CH₃COO⁻]/[CH₃COOH]

[CH₃COO⁻]/[CH₃COOH] = anti log (-0.760)

[CH₃COO⁻]/[CH₃COOH] = 0.174