A 25.00 mL sample of 0.100 M hydrochloric acid is titrated with 0.100 M sodium…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 25.00 mL sample of 0.100 M hydrochloric acid is titrated with 0.100 M sodium hydroxide at 25 °C. The laboratory manual suggests using either methyl orange (transition range pH 3.1–4.4) or phenolphthalein (transition range pH 8.2–10.0) as the visual indicator.
Assume ideal behavior of all solutions and negligible temperature dependence of pKa values over the course of the titration.
(a) Describe qualitatively how the pH changes as NaOH is added, identifying the region of the curve in which the equivalence point occurs.
(b) Explain which of the two indicators would allow the endpoint to coincide most closely with the equivalence point for this titration. Include explicit reference to each indicator’s transition range and the predicted pH at equivalence.
(c) For the indicator that is NOT suitable, predict whether the resulting titration would systematically over-estimate or under-estimate the volume of NaOH required to reach equivalence. Justify your prediction in terms of the relative positions of the endpoint and equivalence point on the titration curve.