This question is about the properties and reactions of propanoic acid, C2H5COOHC_2H_5COOHC2H5COOH. Propanoic acid is a weak acid with an acid dissociation constant, KaK_aKa, of 1.34×10−5 mol dm−31.34 \times 10^{-5} \text{ mol dm}^{-3}1.34×10−5 mol dm−3 at 25∘C25^\circ\text{C}25∘C.
A student uses a pH meter to measure the pH of a solution of C2H5COOHC_2H_5COOHC2H5COOH at 25∘C25^\circ\text{C}25∘C. The measured pH is 2.8202.8202.820. Calculate the concentration of propanoic acid in the solution. Give your answer to three significant figures.
Propanoic acid is added to another weak acid, nitrous acid, HNO2HNO_2HNO2 (Ka=4.50×10−4 mol dm−3K_a = 4.50 \times 10^{-4} \text{ mol dm}^{-3}Ka=4.50×10−4 mol dm−3). An equilibrium is set up containing two acid-base pairs. Write the equilibrium equation and identify the conjugate acid-base pairs as A1, B1 and A2, B2.
The student plans to prepare a buffer solution that has a pH of 4.664.664.66. The buffer solution will contain propanoic acid, C2H5COOHC_2H_5COOHC2H5COOH, and sodium propanoate, C2H5COONaC_2H_5COONaC2H5COONa. The student plans to add 5.28 g5.28\text{ g}5.28 g C2H5COONaC_2H_5COONaC2H5COONa to 150 cm3150\text{ cm}^3150 cm3 of 0.600 mol dm−30.600\text{ mol dm}^{-3}0.600 mol dm−3 C2H5COOHC_2H_5COOHC2H5COOH. The student assumes that the volume of the solution does not change. Show by calculation whether, or not, the student's experimental method would produce the required pH of 4.664.664.66. Show all your working.
When the student prepares the buffer solution, the volume of solution increases slightly. Suggest whether the pH of the buffer solution would be the same, greater than, or less than your calculated value. Explain your reasoning.
