A laboratory technician must prepare 250.0 mL of an aqueous buffer solution with a pH…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A laboratory technician must prepare 250.0 mL of an aqueous buffer solution with a pH of 4.75 and a total conjugate acid–base concentration of 0.200 M. Only 0.50 M acetic acid solution and 0.50 M sodium acetate solution are available.
Assume ideal behaviour and 25 °C throughout.
(1) Determine the numerical value of the ratio [CH3COO−]/[CH3COOH] that will give the required pH. The pKa of acetic acid is 4.76 at 25 °C.
(2) Using your answer from part (a), calculate the moles of CH3COOH and CH3COO− that must be present in the final 250.0 mL of buffer.
(3) Determine the volumes, in millilitres, of the 0.50 M acetic acid solution and the 0.50 M sodium acetate solution that must be mixed to supply the required amounts of each species.
(4) After the two stock solutions are combined, the total volume is found to be 228 mL. Calculate the volume of distilled water that must be added to reach the target volume of 250.0 mL, and explain briefly why this final dilution has a negligible effect on the buffer pH.