CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A diprotic acid, H₂A, with first and second dissociation constants Ka1=3.9×10−4 and Ka2=4.0×10−8 at 25 °C, is titrated with 0.100 M NaOH. A 50.00 mL portion of 0.0600 M H₂A is used.
A chemist wishes to monitor both equivalence points using visual indicators selected from the following list:
• Methyl orange pH 3.1–4.4
• Bromothymol blue pH 6.0–7.6
• Phenolphthalein pH 8.2–10.0
(a) Derive expressions for the pH at each equivalence point in terms of the given Ka values and show how these expressions simplify under reasonable approximations.
(b) Calculate the pH expected at the first and second equivalence points at 25 °C.
(c) For each equivalence point, choose the best indicator from the list and justify each choice with reference to calculated pH values and indicator transition ranges.
(d) Suppose the chemist mistakenly uses the same indicator for both equivalence points. Explain how this choice could compromise the accuracy of one of the measurements, specifying which one and why.