Q.No11. An aqueous solution contains a mixture of 10-M NaCl and 10-M HCl.
Correct Answer: (c) The solution is not a buffer but its pH is less than 7.0
The aqueous solution of 10M NaCl and 10M HCl contains a strong acid and its neutral salt. This mixture fails buffer criteria due to complete HCl dissociation, yielding a highly acidic pH around -1, far below 7.0.
Buffer Basics
Buffers resist pH changes via weak acid-conjugate base or weak base-conjugate acid pairs. Strong acids like HCl fully ionize, lacking equilibrium for pH stabilization. NaCl dissociates into Na⁺ and Cl⁻ without affecting pH.
Solution Composition
10M HCl provides [H⁺] ≈ 10 M, as it dissociates completely: HCl → H⁺ + Cl⁻. NaCl adds Cl⁻ but no H⁺ or OH⁻ impact. Resulting [H⁺] dominates from HCl. pH = -log(10) = -1 < 7.0.
Option Analysis
- (a) Buffer with pH <7.0: Incorrect; strong acid-salt lacks buffering capacity.
- (b) Buffer with pH >7.0: Incorrect; pH is acidic, not basic.
- (c) Not a buffer but pH <7.0: Correct; no weak pair, high acidity confirmed.
pH Calculation
For strong acids >1M, pH = -log[H⁺]. Here, pH ≈ -1, unaffected by NaCl. Real solutions may deviate slightly due to activity, but remains <0. High molarity (10M) implies concentrated, non-ideal conditions, but principle holds.
