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Proton Transfer Reactions · Paper 2 — Free Chemistry HL Practice Questions

1ChallengeSAQ-LStrong vs weak acids and bases5 marksPaper 2~8 min
A student investigates the acid-base properties of two organic compounds, benzoic acid (C6H5COOH\text{C}_6\text{H}_5\text{COOH}) and phenol (C6H5OH\text{C}_6\text{H}_5\text{OH}). Benzoic acid has a pKa\text{p}K_a of 4.204.20 at 298 K298\,\text{K}; phenol has a pKa\text{p}K_a of 9.959.95 at 298 K298\,\text{K}. The student prepares separate 0.100 mol dm−30.100\,\text{mol}\,\text{dm}^{-3} aqueous solutions of each compound. The following expressions may be used: pH=−log⁡[H3O+],pKa=−log⁡Ka,Ka=[H3O+][A−][HA],Kw=1.00×10−14 mol2 dm−6\text{pH} = -\log[\text{H}_3\text{O}^+], \quad \text{p}K_a = -\log K_a, \quad K_a = \frac{[\text{H}_3\text{O}^+][\text{A}^-]}{[\text{HA}]}, \quad K_w = 1.00 \times 10^{-14}\,\text{mol}^2\,\text{dm}^{-6}
(a)
State the expression for pKa\text{p}K_a in terms of KaK_a. [1 mark]
(b)
(i) The pH of the 0.100 mol dm−30.100\,\text{mol}\,\text{dm}^{-3} benzoic acid solution is 2.602.60. Calculate the concentration of hydronium ions, [H3O+][\text{H}_3\text{O}^+], in this solution. [1]
(ii) Using your answer to (b)(i), determine the percentage dissociation of benzoic acid in this solution. [1 mark]
(c)
(i) The student adds 10.0 cm310.0\,\text{cm}^3 of 0.100 mol dm−30.100\,\text{mol}\,\text{dm}^{-3} sodium hydroxide solution to 25.0 cm325.0\,\text{cm}^3 of the 0.100 mol dm−30.100\,\text{mol}\,\text{dm}^{-3} benzoic acid solution (Ka=6.31×10−5 mol dm−3K_a = 6.31 \times 10^{-5}\,\text{mol}\,\text{dm}^{-3}). Calculate the pH of the resulting solution. [2]
(ii) State and explain whether the solution formed in (c)(i) is acidic, basic, or neutral. [1 mark]
(d)
The student repeats the procedure in part (c) using 25.0 cm325.0\,\text{cm}^3 of 0.100 mol dm−30.100\,\text{mol}\,\text{dm}^{-3} phenol instead of benzoic acid, adding the same volume of sodium hydroxide solution. Calculate the pH of the resulting phenol buffer solution and evaluate whether the two buffer solutions formed in (c)(i) and (d) would be effective over the same pH range. [2 marks]

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