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Question EXM.1B.SL.TZ0.1
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[F] Chemistry - 2025
Add this question to the selected test
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
1.
[Maximum mark: 12]
EXM.1B.SL.TZ0.1
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.
(a)
The student reported the volumes of titrant used per trial for samples collected each day in the following table:
(a.i)
The candidate used a 25 cm3 burette with an uncertainty of ±0.05 cm3. Comment on the uncertainty recorded for
the titrant volumes.
[1]
Markscheme
Incorrect AND two readings, uncertainty is ±0.1 ✔
(a.ii)
Calculate the initial % content of Fe2+ in the raw spinach, showing your working.
[3]
Markscheme
«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.
(a.iii)
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]
Markscheme
no difference AND uncertainty larger than difference ✔
Accept other explanations referred to overlapping.
(b.i)
Suggest two flaws in the design that could have contributed to the random error in the investigation.
[2]
Markscheme
dilute the titrant to use larger volumes «of titrant» ✔
ensure spinach leaf fragments are the same size ✔
(b.ii)
The student did not standardise the KMnO4 solution used for titration. Suggest what type of error this may have
caused, giving your reasons.
[1]
Markscheme
systematic error AND all values «equally» inaccurate ✔
(c.i)
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]
Markscheme
Oxidised: Fe+2 → Fe+3 ✔
Reduced: Mn7+ → Mn2+ ✔
(c.ii)
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]
Markscheme
systematic error AND lower accuracy ✔
overestimation of [Fe(II)] ✔
Syllabus sections
Tools
Tools » Tool 3: Mathematics
Inquiry
Inquiry » Inquiry 3: Concluding and evaluating
Reactivity 3. What are the mechanisms of chemical change?
Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.2—Electron transfer reactions » Reactivity 3.2.1—Oxidation and reduction can be
described in terms of electron transfer, change in oxidation state, oxygen gain/loss or hydrogen loss/gain. Deduce oxidation states of an atom in a compound or
an ion. Identify the oxidized and reduced species and the oxidizing and reducing agents in a chemical reaction.
Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.2—Electron transfer reactions
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Question EXM.1B.SL.TZ0.2
Select a Test
[F] Chemistry - 2025
Add this question to the selected test
Date
May Example question
Marks available
[Maximum mark: 14]
Reference code
EXM.1B.SL.TZ0.2
Level
SL
Paper
1B
Time zone
TZ0
Command term
Calculate, Determine, Discuss, Estimate, Suggest
Question number
2
Adapted from
N/A
2.
[Maximum mark: 14]
EXM.1B.SL.TZ0.2
Broccoli is a rich source of ascorbic acid (vitamin C). To investigate the importance of storage temperature of ascorbic acid, a
sample of broccoli was stored at different temperatures, from 5 °C (refrigerator temperature) to 75 °C at 10 °C intervals. A simple
colourimetric method based on the reaction of colourless ascorbic acid, AA with a reagent, R, that produces a coloured complex,
AAR, was used to measure the ascorbic acid content.
AA (aq) + R (aq)
→ AAR (aq)
colourless
purple
(a)
A calibration curve with pure ascorbic acid and appropriate amounts of the reagent, R, was prepared by dilutions
with water of an initial aqueous solution of 100 μg/cm−3 ascorbic acid in water, AA (aq)
(a.i)
Calculate the volumes of pure ascorbic acid solution required for each point of the calibration curve; point 4 of
the curve is shown as an example.
[2]
Markscheme
1- 1.0 cm3
2- 4.0 cm3 ✔
3- 8.0 cm3
5- 4.0 cm3 ✔
Award [1] for 2 correct answers
(a.ii)
The resulting calibration curve is shown:
Suggest a range of absorbance values for which this curve can be used to calculate ascorbic acid of broccoli
accurately.
[1]
Markscheme
0.050−1.000 ✔
(a.iii)
Suggest what should be used as a blank for spectrophotometric reading.
[1]
Markscheme
water AND all samples dissolved «in water» ✔
(a.iv)
Discuss why it is important to obtain a value of R2 close to 1 for a calibration curve.
[2]
Markscheme
ensures the line is best-fit ✔
line/equation of the line will be used for quantitation ✔
Accept any other explanations referring to accuracy.
(b)
After 3 days, the broccoli samples were removed from storage and 1.0 g of each sample was blended with 100.0
cm3 of water. The filtered solution was mixed with the reactant in the same proportions as that used for the
calibration curve in a cuvette and measured.
The sample stored at 5 °C showed an absorbance of 0.600. Determine the concentration of ascorbic acid in the
sample solution by interpolation and using the line equation.
interpolation in graph: ..........................................................................................................................
using line equation: .............................................................................................................................
[2]
Markscheme
Interpolation: 9.8 μg cm−3 ✔
using equation: 0.600/0.06283 = 9.55 «μg cm−3» ✔
(c)
Suggest why water was chosen to extract ascorbic acid from the spinach leaves with reference to its structure.
[2]
Markscheme
«ascorbic acid» has multiple −OH/hydroxyl groups ✔
can H-bond with water ✔
Do not accept OH−/hydroxide for M1
(d)
The content of ascorbic acid in broccoli was calculated in mg per 100 g of broccoli for all values of temperature
and plotted.
(d.i)
Estimate the % change in ascorbic acid concentration when stored for 3 days storage at 5 °C and 20 °C.
[1]
Markscheme
Δ % = «95.5 - 50.0/95.5 × 100»= 47.6 % ✔
Accept calculations using line equation
(d.ii)
Estimate how much ascorbic acid will remain after 6 days storage at 20 °C in the same experimental conditions,
stating any assumption made for the calculation.
[3]
Markscheme
assumption: linear decrease ✔
rate «mg 100 g−1 day−1 = 95.5 − 50.0 / 3 » = 15 «mg 100 g−1 day−1» ✔
«95.5 − 15 × 6 = 5.5 «mg 100 g−1 day−1» ✔
Syllabus sections
Tools
Tools » Tool 1: Experimental techniques
Tools » Tool 3: Mathematics
Structure 2. Models of bonding and structure
Inquiry
Inquiry » Inquiry 1: Exploring and designing
Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.8—The nature of the force that exists between molecules is
determined by the size and polarity of the molecules. Intermolecular forces include London (dispersion), dipole-induced dipole, dipole–dipole and hydrogen
bonding. Deduce the types of intermolecular force present from the structural features of covalent molecules.
Structure 2. Models of bonding and structure » Structure 2.2—The covalent model
Inquiry » Inquiry 2: Collecting and processing data
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