This question is about the properties and reactions of benzoic acid, C6H5COOHC_6H_5COOHC6H5COOH. Benzoic acid is a weak acid with an acid dissociation constant, KaK_aKa, of 6.30×10−5 mol dm−36.30 \times 10^{-5} \text{ mol dm}^{-3}6.30×10−5 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 C6H5COOHC_6H_5COOHC6H5COOH at 25∘C25^\circ\text{C}25∘C. The measured pH is 2.5502.5502.550. Calculate the concentration of benzoic acid in the solution. Give your answer to three significant figures.
Benzoic acid is added to another weak acid, hydrofluoric acid, HFHFHF (Ka=6.80×10−4 mol dm−3K_a = 6.80 \times 10^{-4} \text{ mol dm}^{-3}Ka=6.80×10−4 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 4.154.154.15. The buffer solution will contain benzoic acid, C6H5COOHC_6H_5COOHC6H5COOH, and sodium benzoate, C6H5COONaC_6H_5COONaC6H5COONa. The student plans to add 14.4 g14.4\text{ g}14.4 g of C6H5COONaC_6H_5COONaC6H5COONa to 250 cm3250\text{ cm}^3250 cm3 of 0.450 mol dm−30.450\text{ mol dm}^{-3}0.450 mol dm−3 C6H5COOHC_6H_5COOHC6H5COOH. 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 4.154.154.15. 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.
