CCEA A Level Chemistry
CCEA A Level Chemistry 5.3 Volumetric Analysis Exam Questions
Free CCEA A Level Chemistry revision for 5.3 Volumetric Analysis: what you need to know, common mistakes and exam-style questions with answers. Covers iodine-thiosulfate titrations with starch, estimating oxidants with excess potassium iodide, manganate(VII) titrations with iron(II), equations from half-equations and back titrations.
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About CCEA A Level Chemistry 5.3 Volumetric Analysis
Section 5.3 of Unit A2 2 extends the acid-base titrations of AS 1 (1.9) to redox titrations and back titrations. You need to titrate iodine with sodium thiosulfate using starch, and use this to estimate oxidising agents such as hydrogen peroxide and iodate(V) by adding them to excess acidified potassium iodide. You also need titrations of acidified potassium manganate(VII) with iron(II) and other reducing agents, and to deduce the overall equation from half-equations. Back titrations, for example to find the purity of a Group II oxide or carbonate, are also required. This section uses redox (1.7) and is closely linked to the practical assessment in Unit A2 3.
What you need to know
- Iodine-thiosulfate 2S2O3 2- + I2 gives S4O6 2- + 2I-. Titrate until the solution is pale yellow, add starch (blue-black), then continue until the blue-black colour just disappears.
- Estimating oxidants Oxidants release iodine from excess acidified KI, for example IO3- + 5I- + 6H+ gives 3I2 + 3H2O, so 1 mol IO3- is equivalent to 6 mol S2O3 2-. Hydrogen peroxide: H2O2 + 2H+ + 2I- gives I2 + 2H2O.
- Manganate(VII) MnO4- + 8H+ + 5Fe2+ gives Mn2+ + 4H2O + 5Fe3+. No indicator is needed: the end point is the first permanent pale pink colour. Read the top of the meniscus because the solution is dark.
- Acidification Use dilute sulfuric acid. Hydrochloric acid would be oxidised to chlorine by manganate(VII), giving a high titre.
- Back titration Add a known excess of acid to the sample, then titrate the unreacted acid with standard alkali. Moles reacting with the sample = moles added - moles left over.
Common mistakes in 5.3 Volumetric Analysis
- Adding starch at the start of an iodine titration. Add it near the end point, when the solution is pale yellow, so the complex does not form irreversibly.
- Forgetting the 1:6 ratio in iodate titrations. One iodate releases three iodine molecules, each needing two thiosulfate ions.
- Acidifying manganate(VII) with hydrochloric acid. Use dilute sulfuric acid.
- In back titrations, using the moles of alkali as the moles of sample. Subtract the excess from the total acid added first.
- Reading the bottom of the meniscus for potassium manganate(VII). Read the top because the solution is too dark to see the bottom.
Try three 5.3 Volumetric Analysis exam-style questions
1. 25.0 cm3 of potassium iodate(V) solution was added to excess acidified KI. The iodine released needed 24.0 cm3 of 0.100 mol dm-3 sodium thiosulfate. Calculate the concentration of the iodate. (3 marks)
Show the mark scheme answer
Moles thiosulfate = 0.100 x 0.0240 = 2.40 x 10^-3; moles IO3- = 2.40 x 10^-3 / 6 = 4.00 x 10^-4; concentration = 4.00 x 10^-4 / 0.0250 = 0.0160 mol dm-3.
2. 1.20 g of impure calcium carbonate was added to 50.0 cm3 of 0.500 mol dm-3 HCl. The excess acid needed 20.0 cm3 of 0.200 mol dm-3 NaOH. Calculate the percentage purity. (4 marks)
Show the mark scheme answer
HCl added = 0.0250 mol; excess = 0.00400 mol; HCl reacted = 0.0210 mol so CaCO3 = 0.0105 mol; mass = 0.0105 x 100.1 = 1.05 g; purity = 1.05 / 1.20 x 100 = 87.6%.
3. Explain why no indicator is needed in manganate(VII) titrations. (2 marks)
Show the mark scheme answer
MnO4- is purple and Mn2+ is almost colourless; at the end point the first excess of manganate(VII) gives a permanent pale pink colour.
5.3 Volumetric Analysis FAQs
What does CCEA A Level Chemistry 5.3 Volumetric Analysis cover?
It covers iodine-thiosulfate titrations, estimation of oxidants with excess KI, manganate(VII) titrations and back titrations.
Which unit is 5.3 Volumetric Analysis examined in?
It is examined in Unit A2 2: Analytical, Transition Metals, Electrochemistry and Organic Nitrogen Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.
How do I set out redox titration calculations?
Write the balanced equation or chain of ratios first, find moles of the titrant, use the ratio to find moles of the analyte, then scale up for any dilution or aliquot.
