Question EXM.1B.SL.TZ0.1
| Date | May Example question | Marks available | [Maximum mark: 12] | Reference code | EXM.1B.SL.TZ0.1 |
| Level | SL | Paper | 1B | Time zone | TZ0 |
| Command term | Calculate, Comment, Predict, State, Suggest | Question number | 1 | Adapted from | N/A |
A student investigated the effect of storage time on the iron (II) content of raw spinach using a titration with 0.01M KMnO4 in acid medium.
The equation is:
5Fe+2 (aq) + MnO4− (aq) + 8H+ (aq) → 5Fe+3 (aq) + Mn2+ (aq) + 4H2O (l)
The student acquired the spinach and stored it in the dark in a sealed bag at room temperature. She tested the initial Fe(II) content (Day 1) on 3 samples of 2.0 g each and then each day until day 5. The spinach leaves were roughly torn in pieces and placed in a flask with water for 5 minutes and titrated.
The student reported the volumes of titrant used per trial for samples collected each day in the following table:
The candidate used a 25 cm3 burette with an uncertainty of ±0.05 cm3. Comment on the uncertainty recorded for the titrant volumes.
[1]
Incorrect AND two readings, uncertainty is ±0.1 ✔
Calculate the initial % content of Fe2+ in the raw spinach, showing your working.
[3]
«mol» MnO4− «=0.00339 × 0.01» = 3.38 × 10−5 «mol» ✓
«mol MnO4−: mol Fe+2 = 1:5»
«mol» Fe2+ = 1.95 × 10−4 ✓
% Fe2+ «= 1.95 × 10−4 × 55.8 × 100/2.0»= 0.47«%» ✓
Must show working for the marks.
The results calculated for the subsequent days are shown.
Comment on the significance of the difference in Fe2+ content measured for day 4 and 5.
[1]
no difference AND uncertainty larger than difference ✔
Accept other explanations referred to overlapping.
Suggest two flaws in the design that could have contributed to the random error in the investigation.
[2]
dilute the titrant to use larger volumes «of titrant» ✔
ensure spinach leaf fragments are the same size ✔
The student did not standardise the KMnO4 solution used for titration. Suggest what type of error this may have caused, giving your reasons.
[1]
systematic error AND all values «equally» inaccurate ✔
The titration of Fe(II) with MnO4 in acid medium is a redox reaction. State the oxidised and reduced species, including their change in oxidation states.
Oxidised: ........................................................................................................................................
Reduced: ........................................................................................................................................
[2]
Oxidised: Fe+2 → Fe+3 ✔
Reduced: Mn7+ → Mn2+ ✔
Spinach contains a large amount of antioxidant compounds, including ascorbic acid and oxalic acid. Predict how this will affect the accuracy of the results, mentioning the type and direction of the error.
[2]
systematic error AND lower accuracy ✔
overestimation of [Fe(II)] ✔