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Question
Which mixtures act as buffer solutions?
I. \({\text{100 c}}{{\text{m}}^{\text{3}}}{\text{ 0.1 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}\) ethanoic acid and \({\text{100 c}}{{\text{m}}^{\text{3}}}{\text{ 0.1 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}\) sodium ethanoate
II. \({\text{100 c}}{{\text{m}}^{\text{3}}}{\text{ 0.1 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}\) ethanoic acid and \({\text{50 c}}{{\text{m}}^{\text{3}}}{\text{ 0.1 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}\) sodium hydroxide
III. \({\text{100 c}}{{\text{m}}^{\text{3}}}{\text{ 0.1 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}\) ethanoic acid and \({\text{100 c}}{{\text{m}}^{\text{3}}}{\text{ 0.5 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}\) sodium hydroxide
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
Answer/Explanation
Markscheme
A
Question
A buffer solution is formed by mixing equal volumes of \({\text{1.00 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}\) propanoic acid and \({\text{0.500 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}\) potassium propanoate.
What is the concentration, in \({\text{mol}}\,{\text{d}}{{\text{m}}^{ – 3}}\), of \({\text{[}}{{\text{H}}^ + }{\text{(aq)]}}\) in this buffer solution? (\({K_{\text{a}}}\) for propanoic acid is \(1.30 \times {10^{ – 5}}\).)
A. \(2.60 \times {10^{ – 5}}\)
B. \(1.95 \times {10^{ – 5}}\)
C. \(1.30 \times {10^{ – 5}}\)
D. \(0.650 \times {10^{ – 5}}\)
Answer/Explanation
Markscheme
A
Examiners report
This proved to be answered correctly by fewest candidates (37.59%) with many choosing option D, reversing [HA] and \({\text{[}}{{\text{A}}^ – }{\text{]}}\).
Question
Which mixture will form a buffer in aqueous solution?
A. \({\text{0.10 mol N}}{{\text{H}}_{\text{3}}} + {\text{0.20 mol HCl}}\)
B. \({\text{0.10 mol N}}{{\text{H}}_{\text{3}}} + {\text{0.20 mol NaOH}}\)
C. \({\text{0.10 mol NaOH}} + {\text{0.20 mol KCl}}\)
D. \({\text{0.20 mol N}}{{\text{H}}_{\text{3}}} + {\text{0.10 mol HCl}}\)
Answer/Explanation
Markscheme
D
Question
The \({\text{p}}{K_{\text{a}}}\) of ethanoic acid is 4.8 at 298 K. Which combination will produce a buffer solution with a pH of 4.8 at 298 K?
A. \({\text{20.0 c}}{{\text{m}}^{\text{3}}}{\text{ of 1.0 mol}}\,{\text{d}}{{\text{m}}^{ – {\text{3}}}}{\text{ C}}{{\text{H}}_{\text{3}}}{\text{COOH and 10.0 c}}{{\text{m}}^{\text{3}}}{\text{ of 1.0 mol}}\,{\text{d}}{{\text{m}}^{ – {\text{3}}}}{\text{ NaOH}}\)
B. \({\text{20.0 c}}{{\text{m}}^{\text{3}}}{\text{ of 1.0 mol}}\,{\text{d}}{{\text{m}}^{ – {\text{3}}}}{\text{ C}}{{\text{H}}_{\text{3}}}{\text{COOH and 20.0 c}}{{\text{m}}^{\text{3}}}{\text{ of 1.0 mol}}\,{\text{d}}{{\text{m}}^{ – {\text{3}}}}{\text{ NaOH}}\)
C. \({\text{10.0 c}}{{\text{m}}^{\text{3}}}{\text{ of 1.0 mol}}\,{\text{d}}{{\text{m}}^{ – {\text{3}}}}{\text{ C}}{{\text{H}}_{\text{3}}}{\text{COOH and 20.0 c}}{{\text{m}}^{\text{3}}}{\text{ of 1.0 mol}}\,{\text{d}}{{\text{m}}^{ – {\text{3}}}}{\text{ NaOH}}\)
D. \({\text{14.8 c}}{{\text{m}}^{\text{3}}}{\text{ of 1.0 mol}}\,{\text{d}}{{\text{m}}^{ – {\text{3}}}}{\text{ C}}{{\text{H}}_{\text{3}}}{\text{COOH and 10.0 c}}{{\text{m}}^{\text{3}}}{\text{ of 1.0 mol}}\,{\text{d}}{{\text{m}}^{ – {\text{3}}}}{\text{ NaOH}}\)
Answer/Explanation
Markscheme
A