Three phosphate buffer solutions are adjusted to pH 7.20 at 25 °C using the…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Three phosphate buffer solutions are adjusted to pH 7.20 at 25 °C using the H₂PO₄⁻/HPO₄²⁻ conjugate pair (pKₐ = 7.21).
The compositions are:
• Buffer E: 0.05 M total phosphate (sum of the two species)
• Buffer F: 0.10 M total phosphate
• Buffer G: 0.50 M total phosphate
All three buffers have the same H₂PO₄⁻ : HPO₄²⁻ ratio prior to use.
A student adds exactly 0.0020 mol of NaOH(s) to 200 mL of each buffer, allowing the solid to dissolve completely.
(1) For each buffer, calculate the initial moles of H₂PO₄⁻ and HPO₄²⁻ present in the 200 mL sample.
(2) After the NaOH dissolves, determine the new mole ratio of H₂PO₄⁻ to HPO₄²⁻ for each buffer.
(3) Calculate the new pH of each buffer solution. Then rank the buffers from most to least effective in resisting the pH change caused by the NaOH addition, justifying your ranking quantitatively.