Question

An acidic buffer is obtained on mixing :

• A. 100 mL of 0.1 M HCl and 200 mL of 0.1 M CH₃COONa
• B. 100 mL of 0.1 M HCl and 200 mL of 0.1 M NaCl
• C. 100 mL of 0.1 M CH₃COOH and 100 mL of 0.1 M NaOH
• D. 100 mL of 0.1 M CH₃COOH and 200 mL of 0.1 M NaOH

Hint:

An acidic buffer needs a weak acid and its conjugate base both present in comparable amounts after the reaction.

Answer 1

The Correct Option is A

Work in millimoles.
(1) HCl = 100 × 0.1 = 10 mmol; CH₃COONa = 200 × 0.1 = 20 mmol. HCl + CH₃COONa → CH₃COOH + NaCl leaves 10 mmol CH₃COOH and 10 mmol CH₃COONa: an acidic buffer.
(2) Strong acid plus NaCl: no buffer.
(3) 10 mmol acid + 10 mmol NaOH: complete neutralisation gives only the salt CH₃COONa.
(4) 10 mmol acid + 20 mmol NaOH: excess strong base, no buffer.
Answer: (1) 100 mL of 0.1 M HCl and 200 mL of 0.1 M CH₃COONa