Titration Practical
CIE A-Level Chemistry 9701 · Iodometric Back-Titration & CuSO₄·xH₂O
In this experiment, you determine the value of x in hydrated copper(II) sulfate, CuSO₄·xH₂O. A known mass of the salt (32.5 g) is dissolved in 1.00 dm³ of solution (FA 4). Because Cu²⁺ ions cannot be titrated directly, an iodometric back-titration is used: excess iodide ions (FA 3) are added to reduce Cu²⁺ to Cu⁺, releasing a stoichiometric amount of iodine. This iodine is then titrated against standard sodium thiosulfate solution (FA 1, 0.150 mol dm⁻³) from the burette, using starch as the indicator. From the titre, you calculate the moles of Cu²⁺ in 25.0 cm³ of FA 4, find its concentration, and work backwards to determine x.
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01 — experiment design
Why two reactions?
Cu²⁺ cannot be titrated directly. An indirect (back) titration is used: Cu²⁺ reacts with I⁻ to produce I₂, which is then titrated with thiosulfate.
2Cu²⁺(aq) + 4I⁻(aq) → 2CuI(s) + I₂(aq) I₂(aq) + 2S₂O₃²⁻(aq) → 2I⁻(aq) + S₄O₆²⁻(aq) Key ratio Combining both equations: 2 mol Cu²⁺ : 1 mol I₂ : 2 mol S₂O₃²⁻ — so mol Cu²⁺ = mol S₂O₃²⁻
Role of each reagent
FA 1 — 0.150 mol dm⁻³ Na₂S₂O₃In burette. Titrant — reacts with I₂ at endpoint.
FA 2 — Dilute H₂SO₄Acidifies solution; prevents Cu²⁺ hydrolysis.
FA 3 — 1.00 mol dm⁻³ KIAdded in excess. Converts all Cu²⁺ to I₂. Exact volume does not matter.
FA 4 — CuSO₄·xH₂O solution25.0 cm³ pipetted into flask. The analyte being determined.
Endpoint recognition
Starch is added late — when the solution turns light brown — not at the start.
1
High [I₂] at the start would form an insoluble starch–iodine complex, giving a sluggish, imprecise endpoint.
2
The blue-black colour disappears at the true endpoint. Adding a drop of starch after should cause no further colour change.
02 — data presentation (part a)
✕ Wrong
| Rough | I | II | III | |
|---|---|---|---|---|
| Start V | 0.0 | 0.0 | 16.65 | 0.0 |
| End V | 18.20 | 16.65 | 52.80 | 16.70 |
| Difference | 18.20 | 16.65 | — | 16.70 |
✕
"Start V / End V" — "V" alone not accepted. Must say "initial / final burette reading".
✕
"Difference" — not accepted. Use "titre" or "volume of FA 1 used/added".
✕
No units — cm³ or /cm³ required in every heading.
✕
Final reading 52.80 — no burette reading may exceed 50.00 cm³.
✓ Correct
| Rough | I | II | III | |
|---|---|---|---|---|
| Initial burette reading / cm³ | 0.00 | 0.00 | 16.65 | 0.00 |
| Final burette reading / cm³ | 18.20 | 16.65 | 33.30 | 16.70 |
| Titre / cm³ | 18.20 | 16.65 | 16.65 | 16.70 |
✓
Full heading names with /cm³ units on every column.
✓
"Titre" used — not "difference". No reading exceeds 50.00 cm³.
Burette precision±0.05 cm³ per reading
Valid last digits.00 .05 .10 .15 … .95
Applies toinitial AND final readings
Does NOT apply tothe titre itself
✕ Wrong readings
| Rough | I | II | III | |
|---|---|---|---|---|
| Initial / cm³ | 0 | 0.0 | 16.65 | 0.1 |
| Final / cm³ | 18.2 | 16.65 | 33.3 | 16.7 |
| Titre / cm³ | 18.2 | 16.65 | 16.65 | 16.60 |
✕
0, 0.0, 18.2, 16.7 — only 1 dp. Burette readings must have 2 dp ending in 0 or 5.
✕
0.1 — ends in 1, not 0 or 5. Cannot be read from a standard burette.
✓ Correct readings
| Rough | I | II | III | |
|---|---|---|---|---|
| Initial / cm³ | 0.00 | 0.00 | 16.65 | 0.00 |
| Final / cm³ | 18.20 | 16.65 | 33.30 | 16.70 |
| Titre / cm³ | 18.20 | 16.65 | 16.65 | 16.70 |
✓
Every burette reading: 2 dp ending in 0 or 5. Titre is a derived difference — no restriction on its last digit.
✕ Not concordant — spread 0.15 cm³
| Rough | I | II | III | |
|---|---|---|---|---|
| Titre / cm³ | 18.20 | 16.90 | 16.75 | 16.75 |
✕
II and III agree (spread 0.00) but I differs by 0.15 cm³ — outside the 0.10 limit. A fourth titration is needed.
✓ Concordant — spread ≤ 0.10 cm³
| Rough | I | II | III | |
|---|---|---|---|---|
| Titre / cm³ | 18.20 | 16.65 ✓ | 16.65 ✓ | 16.70 |
✓
I and II identical (spread 0.00). Ticks indicate selection. Rough titre crossed out — never used in mean.
✓
III (16.70) is within 0.10 of selected pair but two concordant titres are sufficient.
03 — mean calculation (part b)
✕ Wrong approaches
A
Rough titre included: (18.20 + 16.65 + 16.65 + 16.70) / 4 = 17.05 — the rough titre must never be averaged in.
B
No selection shown: Writing the mean without ticks or working — the mark scheme requires evidence of which titres were chosen.
C
Wrong decimal places: Mean written as "16.7" — must be quoted to 2 dp (nearest 0.01 cm³).
D
All integers: If every titre used in the mean is a whole number (e.g. 17, 17), the mark is not awarded — readings are too imprecise.
✓ Correct approach
| Rough | I | II | III | |
|---|---|---|---|---|
| Titre / cm³ | 18.20 | 16.65 ✓ | 16.65 ✓ | 16.70 |
Selected titres16.65 and 16.65
Spread16.65 − 16.65 = 0.00 ≤ 0.10 ✓
Mean(16.65 + 16.65) / 2 = 16.65 cm³
Quoted to2 decimal places ✓
✓
Working shown explicitly. Rough titre crossed out. Mean feeds directly into part (b) and all of part (c).
04 — calculations (part c) — interactive
16.65 cm³
c(i) — n(S₂O₃²⁻) = 0.150 × V / 10000.150 × 16.65 / 1000 = 2.498×10⁻³ mol
c(ii) — n(Cu²⁺) [ratio 1:1 from equations]= 2.498×10⁻³ mol
c(iii) — c(Cu²⁺) = n / 0.0250 dm³0.09990 mol dm⁻³
c(iv) — M(CuSO₄·xH₂O) = 32.5 / c(iii)325.3 g mol⁻¹
mass of H₂O per mol = M − 159.6165.7 g mol⁻¹
x = water mass / 18 → round to integerx = 9 (raw: 9.21)
n(S₂O₃²⁻)
2.498×10⁻³
mol
n(Cu²⁺)
2.498×10⁻³
mol
c(Cu²⁺)
0.09990
mol dm⁻³
x
9
integer
Target Drag the slider to 16.65 cm³ to get x = 5, the expected answer for this experiment. All intermediate values should be given to 3–4 sf.
05 — error analysis (part d)
Titre (burette)
Two readings per titre → uncertainty is doubled
uncertainty = 0.05 + 0.05 = 0.10 cm³ % error = (0.10 / titre) × 100 For titre = 16.65 cm³:
= (0.10 / 16.65) × 100 = 0.60%
Pipette (25.0 cm³)
One operation only → single uncertainty
uncertainty = 0.05 cm³ (not doubled) % error = (0.05 / 25.0) × 100 Fixed value regardless of volume delivered:
= 0.20%
Avoid Do not double the pipette uncertainty to 0.10 cm³ by analogy with the burette. The pipette has only one reading — fill to the mark and release. There is no second reading to add.
d(ii) — would a burette for FA 3 improve accuracy?
✓
Why the suggestion makes sense: A burette (±0.05 cm³) is more precise than a measuring cylinder (±0.5 cm³), so the volume of FA 3 would be more precisely known.
✕
Why it would NOT improve accuracy: FA 3 (KI) is used in excess. Its exact volume does not affect the result — any excess converts all Cu²⁺ to I₂. Precisely measuring an excess reagent is meaningless.