A biochemistry laboratory requires 500.0 mL of a pH 4.75 buffer solution that can…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A biochemistry laboratory requires 500.0 mL of a pH 4.75 buffer solution that can neutralize up to 0.0100 mol of added hydrochloric acid without the pH falling below 4.60. The laboratory has glacial acetic acid (density = 1.05 g mL−1; 17.4 M) and solid sodium acetate trihydrate, NaCH3COO·3H2O (molar mass = 136 g mol−1). The acid dissociation constant of acetic acid at 25 °C is Ka=1.8×10−5.
Assume that volumes are additive and that temperature remains at 25 °C throughout.
(a) Calculate the ratio [CH3COOH][CH3COO−] required to achieve a pH of 4.75.
(b) The total analytical concentration of acetic acid species ([CH3COOH] + [CH3COO−]) in the buffer is to be 0.250 M. Determine the number of moles of CH3COOH and CH3COO− needed in the final 500.0 mL of solution.
(c) Calculate (i) the volume of glacial acetic acid and (ii) the mass of sodium acetate trihydrate that must be combined, then diluted to 500.0 mL, to prepare the buffer described in part (b). Report each quantity to three significant figures.
(d) Show quantitatively whether this buffer can neutralize the required 0.0100 mol of added HCl without the pH dropping below 4.60. Support your conclusion with appropriate calculations.