Question

Consider a buffer composed of the weak acid acetic acid (CH₃COOH) and the conjugate base sodium acetate (NaCH₃COO). Which pair of concentrations results in the most effective buffer (i.e. has the highest buffer capacity)?

A. 0.10 M CH₃COOH; 0.10 M NaCH₃COO

B. 0.90 M CH₃COOH; 0.10 M NaCH₃COO

C. 0.10 M CH₃COOH; 0.90 M NaCH₃COO

D. 0.50 M CH₃COOH; 0.50 M NaCH₃COO

Answer 1

Henderson equation pH = pKa + log [Base / Acid]

for most effective buffer pH = pKa

to get pH = pKa , log [Base/Acid ] = or [Base / Acid ] =

for option D. 0.50 M CH₃COOH; 0.50 M NaCH₃COO

[NaCH3COO] / [CH3COOH ] = 0.5/ 0.5 = 1

so D will have greater buffer capacity

Answer : D. 0.50 M CH₃COOH; 0.50 M NaCH₃COO