A research chemist must prepare a standard solution of a monoprotic weak acid, HA
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A research chemist must prepare a standard solution of a monoprotic weak acid, HA, for quality-control analyses. The acid has a Ka of 4.0×10−6. To confirm the concentration, the chemist plans to titrate 50.00mL of the HA solution with 0.150M KOH.
(1) Part 1: (a) Write the balanced molecular equation for the reaction that occurs during the titration.
(2) Part 2: (b) Derive an expression for the pH at the equivalence point in terms of Ka and the concentrations of species present, and then calculate the expected pH numerically.
(3) Part 3: (c) Select an appropriate indicator from the following list and justify your choice using quantitative reasoning:
• Phenolphthalein (pH 8.2–10.0)
• Bromothymol blue (pH 6.0–7.6)
• Methyl orange (pH 3.1–4.4)
(4) Part 4: (d) Suppose phenolphthalein is mistakenly used instead of your recommended indicator. Predict qualitatively how this choice will bias the calculated molarity of HA (too high, too low, or unaffected) and provide a chemical explanation.
(5) Part 5: (e) Propose one additional experimental step, beyond indicator selection, that would further reduce the discrepancy between the end point and equivalence point in this titration, and justify its effectiveness.