Chemistry
Chemistry
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. a. 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. b. 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. c. 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.Intuitive explanation
The key to understanding buffer response is the Henderson-Hasselbalch equation and how the ratio of conjugate base to weak acid determines pH and buffering capacity. Buffer A has equal molar amounts of the acid and base forms, placing it right at the for maximum buffering; Buffer B has less conjugate base, so it starts at a lower pH and has less capacity to neutralize added acid before the ratio shifts significantly. When HCl is added, it consumes the conjugate base (), shifting the equilibrium—the buffer with more base reserves will resist pH change better.
Intuitive explanation
The key to understanding buffer response is the Henderson-Hasselbalch equation and how the ratio of conjugate base to weak acid determines pH and buffering capacity. Buffer A has equal molar amounts of the acid and base forms, placing it right at the for maximum buffering; Buffer B has less conjugate base, so it starts at a lower pH and has less capacity to neutralize added acid before the ratio shifts significantly. When HCl is added, it consumes the conjugate base (), shifting the equilibrium—the buffer with more base reserves will resist pH change better.