Q3(a). pH at the equivalence points of H₃PO₄ with KOH
50 mL of 0.1 M phosphoric (V) acid is being titrated with 0.1 M potassium hydroxide solution at 25 °C. Calculate the pH values at (i) the first equivalence point, (ii) the second equivalence point, and also at (iii) the third equivalence point. Draw the titration curve. Suggest suitable indicator(s) in each stage of this titration. [Given: K₁ = primary dissociation constant of phosphoric (V) acid = 7.5 × 10⁻³; K₂ = secondary dissociation constant = 6.2 × 10⁻⁸; K₃ = tertiary dissociation constant = 5.0 × 10⁻¹³; ionic product of water at 25 °C = 1.1 × 10⁻¹⁴]
Answer in brief
(i) pH = 4.67, (ii) pH = 9.75, (iii) pH = 12.13; indicators: bromocresol green (or methyl orange) for I, thymolphthalein (or phenolphthalein) for II, none for III.
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Coversphosphoric acid titrationpolyprotic acidequivalence point pHtitration curveamphiproticindicator choice
All parts of Question 3
- 3(a) pH at the equivalence points of H₃PO₄ with KOH15 marks
- 3(b) Salt hydrolysis; sodium acetate, 0.01 M15 marks