← 9701/31·ChemistryA-LevelEXAM MODE

Paper 3 Advanced Practical Skills 1

9701/31 · October/November 2025

Question 10/19 answered · 40 marks total

PRACTICAL

Question 1 — Redox titration: ethanedioic acid vs manganate(VII)

Heat FA 1 with dilute sulfuric acid to about 70°C, then titrate against acidified potassium manganate(VII) to a self-indicating pale pink end-point, and use the stoichiometry to find x in (COOH)₂·xH₂O.

[15]

RUN IT ON THE BENCH

Your readings feed straight into the answer table below as you record them.

ON YOUR BENCH — FROM THE CONFIDENTIAL INSTRUCTIONS

Question 1 — solutions

FA 1(150 cm³)
FA 2(150 cm³)
FA 3(100 cm³)

Question 1 — apparatus

25 cm³ pipette(1)
pipette filler(1)
50 cm³ burette(1)
burette stand and clamp(1)
funnel(1)
150 cm³ or 250 cm³ conical flask(2)
25 cm³ measuring cylinder(1)
100 cm³ beaker(1)
white tile(1)
thermometer(1)
tripod(1)
gauze(1)
Bunsen burner and means to light it(1)
heat-proof mat(1)
wash bottle of distilled water(1)

THE BENCH — 25.0 cm³ FA 1 + FA 3, HEATED · FA 2 (ACIDIFIED KMnO4) IN THE BURETTE — ROUGH TITRATION

BURETTE READING

0.35 cm³

initial was 0.35

FLASK TEMP

20°C

FLASK

COLOURLESS (not yet heated)

Why does the purple flash and then fade — until it suddenly doesn't?

While oxalic acid remains in the flask, every drop of MnO₄⁻ that lands is reduced to near-colourless Mn²⁺ almost as fast as it arrives — a purple flash that fades as you swirl it in. The moment the oxalic acid runs out, the next drop has nothing left to reduce it: that drop's colour stays, and the whole flask turns and holds a pale pink. KMnO₄ is its own indicator here — nothing needs to be added.

Why heat to about 70°C, and not just react at room temperature or boil it?

At room temperature the reaction between MnO₄⁻ and ethanedioate ions is slow to start — it only speeds up once some Mn²⁺ has formed, which then catalyses the rest (it's autocatalytic). Warming the flask gives the reaction a head start. Boiling is avoided because it would decompose ethanedioic acid and boil off some of the solution, throwing off the very titre you're trying to measure.

Why add FA 3 (dilute sulfuric acid) at all?

The equation needs H⁺ on the left: 2MnO₄⁻ + 5(COOH)₂ + 6H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O. Without enough acid present, manganate(VII) is reduced instead to brown manganese(IV) oxide, MnO₂ — a different reaction with a different, unusable stoichiometry. FA 3 keeps the solution acidic enough that Mn²⁺ is the only product forming.

What if I overshoot?

A flask that's gone from pale pink to a clear, deep purple means you've added MnO₄⁻ well past the point where any oxalic acid remained — the titre reads high and the run doesn't count. Take a fresh flask, heat it again, run in quickly to just under your rough titre, then go drop by drop.

YOUR TASKS · 0/4

✓Heat FA 1 + FA 3 to ~70°C, then run a rough titration
✓Run accurate titration 1 to a permanent pale pink
✓Run accurate titration 2 to a permanent pale pink
✓Get two concordant accurate titres (within 0.10 cm³)

LAB NOTEBOOK — YOUR TITRES

	ROUGH	ACC. 1	ACC. 2
initial reading /cm³	0.35	0.60	0.25
final reading /cm³	—	—	—
titre /cm³	—	—	—
verdict	—	—	—

YOUR ANSWERS

1(a)[7]

Perform a rough titration of FA 1 (heated with FA 3) against FA 2, then carry out as many accurate titrations as you need for consistent results. Record all your burette readings and titres.

	Rough titration	Accurate titration 1	Accurate titration 2
initial burette reading /cm³
final burette reading /cm³
titre / volume of FA 2 added /cm³

1(b)[1]

From your accurate titration results, calculate a suitable mean value to be used in your calculations: 25.0 cm³ of FA 1 required ___ cm³ of FA 2.