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Context for question 2

Ozone, O

_{3}

(g), is a pollutant at ground level. Some experiments are done to determine the concentration of O

_{3}

(g) in the air outside a school at different times of day. The method involves bubbling a sample of air through a solution of acidified iodide ions, I

^{-}

(aq). Any O

_{3}

(g) in the sample of air reacts to form triiodide ions, I

_{3}^{-}

(aq).

O_{3} (g) + 3 I^{-} (a q) + 2 H^{+} (a q) \to I_{3}^{-} (a q) + O_{2} (g) + H_{2} O (l)

When starch solution is added to the mixture, it forms a coloured complex with I

_{3}^{-}

(aq) ions. The concentration of I

_{3}^{-}

(aq) is found by colorimetry and the percentage of O

_{3}

(g) in the air is determined. A diagram of the colorimeter used is shown in Figure 2.1.

Figure 2.1, a colorimeter: a tall black box, labelled 'black box', containing a 10 cm$^{3}$ measuring cylinder holding 10 cm$^{3}$ of solution, with an LED (light-emitting diode) at the top shining down through the solution and a photocell at the bottom of the box, connected by two wires leaving the box

In the colorimeter, light from the LED passes through the solution in the measuring cylinder. Some of the light is absorbed by the complex containing starch and I

_{3}^{-}

ions. The photocell is used to determine the absorbance of the solution in the measuring cylinder.

2(a)Uv Visible SpectroscopyEasy1 mark

Suggest one reason why the colorimeter must be enclosed in a black box.

Context for 2(b)

To calibrate the colorimeter shown in Figure 2.1, five solutions of different known concentrations of I

_{3}^{-}

(aq) are prepared. For each solution, the following procedure is carried out. step 1 Place 15.0 cm

^{3}

of I

_{3}^{-}

(aq) solution in a boiling tube. step 2 Add a few drops of starch solution to the boiling tube so that I

_{3}^{-}

(aq) ions form a coloured complex. step 3 Wait for ten minutes. step 4 Transfer 10.0 cm

^{3}

of solution from the boiling tube to a 10 cm

^{3}

measuring cylinder. step 5 Place the measuring cylinder into the colorimeter. step 6 Switch on the LED and record the resistance of the photocell. The known concentrations of I

_{3}^{-}

(aq) used and the resistance readings of the photocell,

R_{s}

, are shown in Table 2.1. The absorbance,

A

, of each solution is calculated using equation 1:

A = lo g (\frac{R _{s}}{R _{0}}) (equation 1)

R_{0}

is the resistance measured in a control experiment. The value of

R_{0}

in this experiment is 4.85 kΩ.

2(b)(i)Uv Visible SpectroscopyEasy1 mark

Complete Table 2.1. Record your values to three significant figures.

concentration of I $_{3}^{-}$ (aq) / mol dm $^{- 3}$	resistance of photocell, $R_{s}$ / kΩ	absorbance, $A$
$3.60 \times 1 0^{- 6}$	33.46	Table answer r1_absorbance, $A$
$2.94 \times 1 0^{- 6}$	23.42	Table answer r2_absorbance, $A$
$2.20 \times 1 0^{- 6}$	16.01	Table answer r3_absorbance, $A$
$1.52 \times 1 0^{- 6}$	9.68	Table answer r4_absorbance, $A$
$7.90 \times 1 0^{- 7}$	7.50	Table answer r5_absorbance, $A$

2(b)(ii)Uv Visible SpectroscopyEasy1 mark

Suggest what was placed in the measuring cylinder in Figure 2.1 to obtain the value for

R_{0}

.

Answer

0 words

2(b)(iii)Experimental PlanningEasy1 mark

State the independent variable.

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

Use the results from Table 2.1 to plot a graph on the grid in Figure 2.2 to show the relationship between absorbance,

A

, and the concentration of I

_{3}^{-}

(aq). Use a cross (

\times

) to plot each data point. Draw a line of best fit.

Draw with the mouse or a finger.

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

Circle the most anomalous point on the graph in Figure 2.2. Suggest one reason to explain the anomalous point you have circled. Assume no error was made in preparing the I

_{3}^{-}

(aq) solution.

Draw with the mouse or a finger.

Context for 2(c)

The following procedure is used to determine the concentration of O

_{3}

(g) in each sample of air. step 1 Bubble air through a boiling tube containing 15.0 cm

^{3}

of acidified aqueous iodide ions, I

^{-}

(aq), for 30 minutes, as shown in Figure 2.3.

Figure 2.3, a boiling tube containing 15.0 cm$^{3}$ of acidified I$^{-}$(aq), with two narrow tubes passing through a bung in its mouth labelled 'air in' (arrow pointing down into the tube) and 'air out' (arrow pointing up out of the tube), the air-in tube extending down near the liquid surface with small bubbles rising through the liquid, labelled 'boiling tube'

Repeat steps 2–6 from the procedure in 2(b).

2(c)(i)Practical Techniques SafetyEasy1 mark

Suggest a reason for waiting ten minutes after the starch solution is added (step 3).

2(c)(ii)Practical Techniques SafetyEasy1 mark

Identify the most appropriate piece of apparatus to transfer 10.0 cm

^{3}

of the solution from the boiling tube to a 10 cm

^{3}

measuring cylinder (step 4).

2(c)(iii)Uv Visible SpectroscopyMedium2 marks

A student follows the procedure and measures the photocell resistance,

R_{s}

, as 21.17 kΩ. Use this information and your graph in Figure 2.2 to find the concentration, in mol dm

^{- 3}

, of I

_{3}^{-}

(aq).

absorbance,

A

=

concentration of I

_{3}^{-}

(aq) =

0 words

2(c)(iv)Concentration Solutions Titration CalculationsEasy1 mark

Use your answer to 2(c)(iii) to calculate the amount, in mol, of O

_{3}

(g) in the air bubbled through the boiling tube.

amount of O

_{3}

(g) =

0 words

2(c)(v)Concentration Solutions Titration CalculationsEasy1 mark

In step 1 in 2(c), the volume of air bubbled through the boiling tube in 30 minutes is 7500 cm

^{3}

. Use your answer to 2(c)(iv) to calculate the concentration, in mol dm

^{- 3}

, of O

_{3}

(g) in the air. If you were unable to determine an answer to 2(c)(iv), then use amount of O

_{3}

(g) as

3.23 \times 1 0^{- 8}

mol. This may not be the correct answer.

concentration of O

_{3}

(g) =

0 words

2(c)(vi)Uncertainty Errors EvaluationHard1 mark

A student draws the line of best fit on the calibration graph that has a gradient which is too low. State the effect, if any, on the calculated value for the concentration of O

_{3}

(g). Explain your answer.

2(c)(vii)Uncertainty Errors EvaluationMedium1 mark

The procedure is repeated at different times of day. The resistances of the photocell are shown in Table 2.2.

Table 2.2

time of day	resistance of photocell, $R_{s}$ / kΩ
08:00	9.87
12:00	13.66

The total percentage error of each of the resistance readings is determined to be 14.0%. Use this value and the data from Table 2.2 to determine whether the difference in the readings at 08:00 and 12:00 can be accounted for by measurement error or whether there is a difference in the concentration of O

_{3}

(g) in the air. Show your working.

Answer

0 words

2(c)(viii)Uv Visible SpectroscopyHard1 mark

The procedure is repeated using a different type of 10 cm

^{3}

measuring cylinder. The 10 cm

^{3}

measuring cylinder used is narrower and taller than that used in the first experiment. State the effect, if any, this would have on the value obtained for the resistance of the photocell,

R_{s}

. Explain your answer.

2(c)(ix)Uncertainty Errors EvaluationEasy1 mark

The procedure is repeated but air is bubbled for more than 30 minutes in step 1. Suggest one reason why it is better to bubble air through the solution for longer.

May/June 2026 Paper 51

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