A researcher prepares two separate phosphate buffers
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A researcher prepares two separate phosphate buffers, each at 25 °C and a total volume of 500 mL.
• Buffer A is composed of 0.20 mol of dihydrogen phosphate ion (H₂PO₄⁻) and 0.20 mol of hydrogen phosphate ion (HPO₄²⁻).
• Buffer B is composed of 0.20 mol of dihydrogen phosphate ion and 0.10 mol of hydrogen phosphate ion.
The researcher wishes to understand how each buffer responds to the addition of a small amount of strong acid. In separate trials, 1.0 mL of 0.50 M HCl is added to each buffer. Assume the volume change is negligible.
Use qualitative reasoning and any appropriate quantitative estimates to answer the following.
(1) Predict which buffer, A or B, will have the smaller pH change after the HCl addition. Clearly justify your answer in terms of buffer composition.
(2) Estimate whether the final pH of Buffer A after the acid addition will be greater than, less than, or approximately equal to 7.2 (the pH at which the dihydrogen phosphate/hydrogen phosphate pair is in a 1:1 ratio). Explain your reasoning.
(3) If an equivalent volume of 0.50 M NaOH were added to fresh samples of each buffer instead of HCl, discuss how the relative pH shifts would compare to those observed for acid addition. Provide a justification that considers the direction of reaction with the buffer components.