A student wishes to prepare 250.0 mL of a phosphate buffer at pH 7.20
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A student wishes to prepare 250.0 mL of a phosphate buffer at pH 7.20, using only the 0.100 M stock solutions available in the teaching laboratory: NaH₂PO₄(aq) and Na₂HPO₄(aq). Unfortunately, only 25.0 mL of the Na₂HPO₄ stock solution remains, whereas an unlimited supply of NaH₂PO₄ is on hand. Assume pKa (second dissociation) of phosphoric acid is 7.21 at 25 °C.
(a) Calculate the [H2PO4−][HPO42−] ratio required for the buffer to have a pH of 7.20.
(b) Determine the total number of moles of HPO42− that would be needed in 250.0 mL if the student aims for a combined buffer concentration of 0.300 M (i.e., [H2PO4−]+[HPO42−]=0.300M).
(c) Using the maximum 25.0 mL of 0.100 M Na₂HPO₄ available, calculate the highest attainable concentration of HPO42− in the final 250.0 mL solution.
(d) Based on your calculations in parts (b) and (c), evaluate quantitatively whether the student can meet both the pH target and the 0.300 M total concentration requirement simultaneously. Support your conclusion with numerical evidence.
(e) If the requirements cannot be met, propose one practical adjustment (other than obtaining more Na₂HPO₄) that would allow the student to achieve the desired pH using the materials at hand, and briefly explain how this adjustment resolves the limitation.