Samples of two phosphate buffers are prepared at 25 °C
CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Samples of two phosphate buffers are prepared at 25 °C. Each sample has a volume of 200.0 mL and an initial pH of 7.21.
• Buffer E: 0.250 M H₂PO₄⁻ and 0.250 M HPO₄²⁻
• Buffer F: 0.050 M H₂PO₄⁻ and 0.050 M HPO₄²⁻
A chemist adds 5.00 mL of 0.200 M NaOH to each buffer and stirs thoroughly.
Which buffer will experience the smaller change in pH, and why?
(a) Buffer F, because its acid–to–base ratio is closer to 1:1 after the NaOH is added.
(b) Buffer E, because the common-ion effect suppresses additional dissociation of H₂PO₄⁻, minimizing the pH change.
(c) Buffer E, because it contains the greater absolute concentrations of both buffer components.
(d) Buffer F, because the added OH⁻ is more highly diluted in the less-concentrated buffer, so the change in pH is smaller.