Proper understanding [74 marks]
1. 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 KMnO 4 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:
[[N/A]]
(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 Fe 2+ 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)] ✔
2. 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)
colourless
(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)
[[N/A]]
(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 R 2 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.
[[N/A]]
(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» ✔
3. EXM.1B.HL.TZ0.1
A student investigated the effect of storage time on the iron (II) content
of raw spinach using a titration with 0.01M KMnO 4 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:
[[N/A]]
(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 Fe 2+ 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)] ✔
4. EXM.1B.HL.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)
colourless
(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)
[[N/A]]
(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 R 2 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.
[[N/A]]
(b.i)
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» ✔
(b.ii)
Suggest which of the two methods will provide a more accurate value.
[1]
Markscheme
line equation AND uses the values for 5 determinations ✔
(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.
[[N/A]]
(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» ✔
5. 20N.1B.SL.TZ0.a
Suggest why a non-polar solvent was needed.
[1]
Markscheme
oil is non-polar «and dissolves best in non-polar solvents»
OR
oil does not dissolve in polar solvents ✔ Do not accept “like dissolves like”
only.
6. 20N.1B.SL.TZ0.a(i)
Proteins are polymers of amino acids.
A paper chromatogram of two amino acids, A1 and A2, is obtained using
a non-polar solvent.
© International Baccalaureate Organization 2020.
Determine the R f value of A1.
[1]
Markscheme
0.70 ✔ Accept any value within the range “0.67 − 0.73”.
7. 20N.1B.SL.TZ0.a(i)
Proteins are polymers of amino acids.
A paper chromatogram of two amino acids, A1 and A2, is obtained using
a non-polar solvent.
© International Baccalaureate Organization 2020.
Determine the R f value of A1.
[1]
Markscheme
0.70 ✔ Accept any value within the range “0.67 − 0.73”.
8. 20N.1B.SL.TZ0.a
Identify the independent and dependent variables in this experiment.
[1]
Markscheme
Independent variable:
chain length OR number of carbon «atoms in alcohol»
AND
Dependent variable:
volume of N a O H OR K c /equilibrium constant OR equilibrium
concentration/moles of C H 3 C O O H ✔
9. 20N.1B.SL.TZ0.a(i)
Proteins are polymers of amino acids. A paper chromatogram of two
amino acids, A1 and A2, is obtained using a non-polar solvent.
© International Baccalaureate Organization 2020.
Determine the R f value of A1.
[1]
Markscheme
0.70 ✔ Accept any value within the range “0.67 − 0.73”.
10. 20N.1B.SL.TZ0.b
Aspirin crystals are rinsed with water after recrystallization to remove
impurities.
Suggest why cold water is used.
[1]
Markscheme
to avoid dissolving the crystals/aspirin ✔
Accept “to avoid loss of product” OR “aspirin is less soluble in cold water”.
11. 20N.1B.SL.TZ0.b
Aspirin crystals are rinsed with water after recrystallization to remove
impurities.
Suggest why cold water is used.
[1]
Markscheme
to avoid dissolving the crystals/aspirin ✔
Accept “to avoid loss of product” OR “aspirin is less soluble in cold water”.
12. 20N.1B.SL.TZ0.b
Outline a green chemistry solution for problems generated by the use of
organic solvents.
[1]
Markscheme
«use of» alternative solvents such as supercritical/liquid C O2
Do not accept political or regulatory solutions.
OR
use of water «as solvent»
OR
solvent-free reactions «for example, polymerization of propene»
OR
solid-state chemistry
OR
recycle «waste» solvents
OR
catalysis that leads to better/higher yield
“catalysis” alone not sufficient for mark.
OR
reducing number of steps ✔
13. 20N.1B.SL.TZ0.a(ii)
The vapour pressure of pure ethanal at 20❑∘ C is 101 k P a.
Calculate the vapour pressure of ethanal above the liquid mixture at
∘
20❑ C .
[1]
Markscheme
« ρet h a n a l=0.250 ×101=» 25.3 « k P a » ✔
14. 19N.1B.SL.TZ0.a
Draw a best-fit line on the graph.
[1]
Markscheme
best-fit smooth curve ✔
NOTE: Do not accept a series of connected lines that pass through all
points OR any straight line representation.
15. 19N.1B.SL.TZ0.a(i)
Describe the effect of increasing the voltage on the chemical yield of:
Ethanal using Pt/C:
Carbon dioxide using PtRu/C:
[2]
Markscheme
Ethanal using Pt/C:
decreases ✔
Carbon dioxide using PtRu/C:
«generally» increases AND then decreases ✔
NOTE: Accept “no clear trend/pattern” OR “increases and decreases” OR
“increases, reaches a plateau and «then» decreases” for M2.
16. 19N.1B.SL.TZ0.a
Draw a best-fit line on the graph.
[1]
Markscheme
best-fit smooth curve ✔
NOTE: Do not accept a series of connected lines that pass through all
points OR any straight line representation.
17. 19N.1B.SL.TZ0.a(i)
Describe the effect of increasing the voltage on the chemical yield of:
Ethanal using Pt/C:
Carbon dioxide using PtRu/C:
[2]
Markscheme
Ethanal using Pt/C:
decreases ✔
Carbon dioxide using PtRu/C:
«generally» increases AND then decreases ✔
NOTE: Accept “no clear trend/pattern” OR “increases and decreases” OR
“increases, reaches a plateau and «then» decreases” for M2.
18. 19N.1B.SL.TZ0.a(i)
Describe the effect of increasing the voltage on the chemical yield of:
Ethanal using Pt/C:
Carbon dioxide using PtRu/C:
[2]
Markscheme
Ethanal using Pt/C:
decreases ✔
Carbon dioxide using PtRu/C:
«generally» increases AND then decreases ✔
NOTE: Accept “no clear trend/pattern” OR “increases and decreases” OR
“increases, reaches a plateau and «then» decreases” for M2.
19. 19M.1B.SL.TZ1.a(i)
Estimate the time at which the powdered zinc was placed in the beaker.
[1]
Markscheme
100 «s» [✔] Note: Accept 90 to 100 s.
20. 19M.1B.SL.TZ1.a
Deduce, giving a reason, the group of elements in the periodic table
most likely to undergo sublimation.
[2]
Markscheme
group 18/noble gases [✔] smallest difference between melting and
boiling points
OR
weakest intermolecular forces «in that period» [✔] Note: Accept “group
17/halogens”.
21. 19M.1B.SL.TZ1.a(i)
Estimate the time at which the powdered zinc was placed in the beaker.
[1]
Markscheme
100 «s» [✔] Note: Accept 90 to 100 s.
© International Baccalaureate Organization, 2025
