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Rates, equilibrium and pH

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Question 6

Ethanoic acid, CH3COOH\text{CH}_3\text{COOH}CH3​COOH, is a weak Brønsted–Lowry acid with a pKa\text{p}K_{\text{a}}pKa​ value of 4.764.764.76 at room temperature.

CH3COOH\text{CH}_3\text{COOH}CH3​COOH can be prepared by reacting ethanoic anhydride, (CH3CO)2O(\text{CH}_3\text{CO})_2\text{O}(CH3​CO)2​O, with water, where CH3COOH\text{CH}_3\text{COOH}CH3​COOH is the only product:

(CH3CO)2O+H2O→2CH3COOH (\text{CH}_3\text{CO})_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{CH}_3\text{COOH} (CH3​CO)2​O+H2​O→2CH3​COOH

A chemist prepares a buffer solution using the following method:

  • Step 1: The chemist weighs a sample of (CH3CO)2O(\text{CH}_3\text{CO})_2\text{O}(CH3​CO)2​O. Water is then added to form 250 cm3250\text{ cm}^3250 cm3 of 0.800 mol dm−30.800\text{ mol dm}^{-3}0.800 mol dm−3 CH3COOH(aq)\text{CH}_3\text{COOH}(\text{aq})CH3​COOH(aq).
  • Step 2: The chemist adds 200 cm3200\text{ cm}^3200 cm3 of 0.350 mol dm−30.350\text{ mol dm}^{-3}0.350 mol dm−3 KOH(aq)\text{KOH}(\text{aq})KOH(aq) to 150 cm3150\text{ cm}^3150 cm3 of the 0.800 mol dm−30.800\text{ mol dm}^{-3}0.800 mol dm−3 solution of CH3COOH(aq)\text{CH}_3\text{COOH}(\text{aq})CH3​COOH(aq). The resulting solution is made up to 500 cm3500\text{ cm}^3500 cm3 with distilled water.

Explain why a buffer solution forms in Step 2. Determine the pH of this buffer solution (to 2 decimal places) and the mass of (CH3CO)2O(\text{CH}_3\text{CO})_2\text{O}(CH3​CO)2​O (to 3 significant figures) that was used in Step 1.

Take the relative atomic masses as: H=1.0\text{H} = 1.0H=1.0, C=12.0\text{C} = 12.0C=12.0, O=16.0\text{O} = 16.0O=16.0.

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Rates, equilibrium and pH Questions

  1. A Level
  2. /Chemistry
  3. /Rates, equilibrium and pH