DP Chemistry (first assessment 2025)

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Question EXM.1B.SL.TZ0.2

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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» ✔

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