A biochemistry student prepared a model physiological buffer solution by dissolving 0.0250 mol0.0250\text{ mol}0.0250 mol of sodium lactate (NaLac\text{NaLac}NaLac) in 250 cm3250\text{ cm}^3250 cm3 of a 0.300 mol dm−30.300\text{ mol\ dm}^{-3}0.300 mol dm−3 solution of lactic acid (HLac\text{HLac}HLac) and stirring thoroughly.
The student then added 8.00×10−3 mol8.00 \times 10^{-3}\text{ mol}8.00×10−3 mol of hydrochloric acid (HCl\text{HCl}HCl) to the buffer solution. Calculate the pH\text{pH}pH of the resulting buffer solution at 25 ∘C25\ ^\circ\text{C}25 ∘C.
For lactic acid at 25 ∘C25\ ^\circ\text{C}25 ∘C, the acid dissociation constant is Ka=1.38×10−4 mol dm−3K_{\text{a}} = 1.38 \times 10^{-4}\text{ mol\ dm}^{-3}Ka=1.38×10−4 mol dm−3.
Give your answer to two decimal places.
Explain why diluting this lactic acid buffer solution with deionised 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.