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Context for question 2

Blood plasma contains a number of different cations. Many of these cations emit a different colour of light when placed in a flame. Flame emission photometry is a method of determining the concentration of cations in a sample using the intensity of light emitted. In an experiment, a sample of blood plasma is placed into a colourless flame. K

^{+}

ions in the blood plasma absorb energy from the flame and emit light with a lilac colour. The intensity of the light emitted is measured. To determine the concentration of K

^{+}

ions in the sample, a calibration graph showing the relationship between known concentration of K

^{+}

ions and light intensity is required.

Context for 2(a)

A solution of potassium chloride, KCl(aq), is prepared. The concentration of aqueous potassium ions, K

^{+}

, in this solution is 0.100 g dm

^{- 3}

. Different volumes of the solution of KCl(aq) are taken and diluted by adding specific volumes of distilled water to prepare solutions of specific concentrations. Each prepared solution of known [K

^{+}

(aq)] is tested using flame emission photometry. Table 2.1 shows the relative light intensity measured for each prepared solution.

2(a)(i)Concentration Solutions Titration CalculationsEasy1 mark

Complete Table 2.1 to show the volume of distilled water added to make each prepared solution of known [K

^{+}

(aq)].

volume of KCl(aq) / cm $^{3}$	volume of distilled water added / cm $^{3}$	[K $^{+}$ (aq)] / g dm $^{- 3}$	relative light intensity
0.00	50.00	$0.00 \times 1 0^{- 3}$	0.0
0.50	Table answer r2_volume of distilled water added / cm$^{3}$	$1.00 \times 1 0^{- 3}$	2.1
1.00	Table answer r3_volume of distilled water added / cm$^{3}$	$2.00 \times 1 0^{- 3}$	4.2
1.50	Table answer r4_volume of distilled water added / cm$^{3}$	$3.00 \times 1 0^{- 3}$	6.7
2.00	Table answer r5_volume of distilled water added / cm$^{3}$	$4.00 \times 1 0^{- 3}$	8.8
2.50	Table answer r6_volume of distilled water added / cm$^{3}$	$5.00 \times 1 0^{- 3}$	11.0
3.00	Table answer r7_volume of distilled water added / cm$^{3}$	$6.00 \times 1 0^{- 3}$	11.2
3.50	Table answer r8_volume of distilled water added / cm$^{3}$	$7.00 \times 1 0^{- 3}$	15.7
4.00	Table answer r9_volume of distilled water added / cm$^{3}$	$8.00 \times 1 0^{- 3}$	17.1
4.50	Table answer r10_volume of distilled water added / cm$^{3}$	$9.00 \times 1 0^{- 3}$	17.1
5.00	Table answer r11_volume of distilled water added / cm$^{3}$	$10.00 \times 1 0^{- 3}$	17.1

2(a)(ii)Practical Techniques SafetyEasy1 mark

Identify a suitable piece of apparatus for adding the distilled water.

2(a)(iii)Experimental PlanningEasy1 mark

Identify the dependent variable in this experiment.

2(a)(iv)Tables Graphs Data AnalysisMedium2 marks

Plot a calibration graph on the grid in Figure 2.1 to show the relationship between [K

^{+}

(aq)] and relative light intensity. Use a cross (

\times

) to plot each data point. Draw one line of best fit for [K

^{+}

(aq)] from

0.00 \times 1 0^{- 3}

to

7.00 \times 1 0^{- 3}

g dm

^{- 3}

. Draw a second line of best fit for [K

^{+}

(aq)] from

8.00 \times 1 0^{- 3}

to

10.00 \times 1 0^{- 3}

g dm

^{- 3}

. Extend both lines so they meet.

Draw with the mouse or a finger.

2(a)(v)Tables Graphs Data AnalysisEasy1 mark

Describe how the graph shows that [K

^{+}

(aq)] from

0.00 \times 1 0^{- 3}

to

7.00 \times 1 0^{- 3}

g dm

^{- 3}

is directly proportional to relative light intensity.

2(a)(vi)Tables Graphs Data AnalysisMedium1 mark

Circle the most anomalous point on your graph in Figure 2.1. Suggest one reason to explain the anomalous point you have circled.

Draw with the mouse or a finger.

2(b)(i)Tables Graphs Data AnalysisEasy1 mark

A sample of blood plasma is diluted 100 times and tested using flame emission photometry. The relative light intensity recorded is 3.8. Use your graph in Figure 2.1 to determine the [K

^{+}

(aq)], in g dm

^{- 3}

, in the sample.

[K

^{+}

(aq)] in diluted blood plasma

2(b)(ii)Tables Graphs Data AnalysisMedium1 mark

The healthy range for [K

^{+}

(aq)] in blood plasma is between 0.100 and 0.200 g dm

^{- 3}

. State whether the sample of blood plasma tested in 2(b)(i) is within the healthy range, too high or too low. Explain your answer.

Classification and explanation

0 words

2(b)(iii)Uv Visible SpectroscopyMedium1 mark

When testing samples of blood plasma, a coloured light filter needs to be used which only allows lilac-coloured light emitted from the sample to reach the detector. Suggest why this coloured light filter is needed when testing blood plasma but is not necessary when testing KCl(aq).

2(c)Tables Graphs Data AnalysisMedium1 mark

Explain why flame emission photometry is not suitable for measuring [K

^{+}

(aq)] at

8.00 \times 1 0^{- 3}

g dm

^{- 3}

and above.

2(d)Uncertainty Errors EvaluationMedium1 mark

State whether the data used for the calibration graph in Figure 2.1 is reliable. Explain your answer.

February/March 2026 Paper 52

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