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Rates, equilibrium and pH

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Question 3

Propanoic acid, C2H5COOH\text{C}_2\text{H}_5\text{COOH}C2​H5​COOH, is a weak Brønsted–Lowry acid with a pKa\text{p}K_{\text{a}}pKa​ value of 4.874.874.87 at room temperature.

C2H5COOH\text{C}_2\text{H}_5\text{COOH}C2​H5​COOH can be prepared by reacting propanoic anhydride, (C2H5CO)2O(\text{C}_2\text{H}_5\text{CO})_2\text{O}(C2​H5​CO)2​O, with water, where propanoic acid is the only product:

(C2H5CO)2O+H2O→2C2H5COOH (\text{C}_2\text{H}_5\text{CO})_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{C}_2\text{H}_5\text{COOH} (C2​H5​CO)2​O+H2​O→2C2​H5​COOH

A chemist prepares a buffer solution using the following method:

  • Step 1: The chemist weighs a sample of (C2H5CO)2O(\text{C}_2\text{H}_5\text{CO})_2\text{O}(C2​H5​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 C2H5COOH(aq)\text{C}_2\text{H}_5\text{COOH}(\text{aq})C2​H5​COOH(aq).
  • Step 2: The chemist adds 150 cm3150\text{ cm}^3150 cm3 of 0.400 mol dm−30.400\text{ mol dm}^{-3}0.400 mol dm−3 NaOH(aq)\text{NaOH}(\text{aq})NaOH(aq) to 100 cm3100\text{ cm}^3100 cm3 of the 0.800 mol dm−30.800\text{ mol dm}^{-3}0.800 mol dm−3 solution of C2H5COOH(aq)\text{C}_2\text{H}_5\text{COOH}(\text{aq})C2​H5​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 (C2H5CO)2O(\text{C}_2\text{H}_5\text{CO})_2\text{O}(C2​H5​CO)2​O (to 3 significant figures) that was used in Step 1.

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