The method and the results are on the redox reactions lab. Do the analysis yourself first, then check the answers below.
Quick answer: Redox reactions transfer electrons: oxidation is loss of electrons and an increase in oxidation number, reduction is gain of electrons and a decrease. These answers go with the Chemistry 11-12 lab on redox reactions: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
Model answers follow the sample burette readings. Your own titres will differ a little. Judge your lab by the agreement of your concordant results, the correct mole ratio 2 : 5 and a sensible percentage strength.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R3.2
The three results 17.65, 17.70 and 17.60 are within 0.10 cm³ of each other (the rough titre is excluded). Mean = (17.65 + 17.70 + 17.60) ÷ 3 = 17.65 cm³.
Question 2 Why no indicator
Manganate(VII) is intensely purple and its reduced form Mn²⁺ is almost colourless, so it acts as its own indicator. The end point is the first permanent pale pink (excess MnO₄⁻ no longer reacts).
Question 3 Choice of acid
Sulfuric acid supplies the H⁺ without adding a reactive anion. Hydrochloric acid contains Cl⁻, which MnO₄⁻ oxidises to Cl₂, giving a falsely high titre. Nitric acid is itself an oxidising agent and could oxidise the sample.
Ratio MnO₄⁻ : H₂O₂ = 2 : 5, so n(H₂O₂) = 3.53 × 10⁻⁴ × 2.5 = 8.83 × 10⁻⁴ mol in 25.0 cm³. In 250 cm³: 8.83 × 10⁻³ mol, all from 5.00 cm³, so c = 8.83 × 10⁻³ ÷ 0.00500 = 1.77 mol/dm³. Mass = 1.765 × 34.0 = 60.0 g per dm³ = 6.0 g per 100 cm³ (6.0 %).
Question 6 Uncertainty
Two readings are subtracted, so the uncertainties add to ±0.10 cm³. Percentage = 0.10 ÷ 17.65 × 100 = 0.57 %.
Question 7 Improving the method
Read the burette at eye level to avoid parallax, repeat the titration until three results agree within 0.05 cm³, and standardise the manganate(VII) solution against a primary standard before use. The purple fades very slowly for the first drops and then quickly because Mn²⁺ formed acts as a catalyst.
