A biochemist prepares a buffer solution to simulate cellular conditions by dissolving 0.0350 mol0.0350\text{ mol}0.0350 mol of sodium propanoate (C2H5COONa\text{C}_2\text{H}_5\text{COONa}C2H5COONa) in 250 cm3250\text{ cm}^3250 cm3 of a 0.250 mol dm−30.250\text{ mol\ dm}^{-3}0.250 mol dm−3 solution of propanoic acid (C2H5COOH\text{C}_2\text{H}_5\text{COOH}C2H5COOH) and mixing the solution thoroughly.
The biochemist then adds 8.00×10−3 mol8.00 \times 10^{-3}\text{ mol}8.00×10−3 mol of solid potassium hydroxide (KOH\text{KOH}KOH) to the buffer solution, stirring until it completely dissolves. Calculate the pH\text{pH}pH of the resulting buffer solution at 25 ∘C25\ ^\circ\text{C}25 ∘C after the addition of the potassium hydroxide.
For propanoic acid at 25 ∘C25\ ^\circ\text{C}25 ∘C, the acid dissociation constant is Ka=1.35×10−5 mol dm−3K_{\text{a}} = 1.35 \times 10^{-5}\text{ mol\ dm}^{-3}Ka=1.35×10−5 mol dm−3.
Give your answer to two decimal places.
Explain why diluting this buffer solution with water has almost no effect on its pH\text{pH}pH, using a mathematical expression to support your explanation.
77 exam-style questions on AQA A Level Chemistry Acids and bases (A-level only), covering Brønsted–Lowry acid–base equilibria in aqueous solution (A-level only), Definition and determination of pH (A-level only), The ionic product of water, $K_w$ (A-level only), Weak acids and bases $K_a$ for weak acids (A-level only), pH curves, titrations and indicators (A-level only), and Buffer action (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.