O Level Chemistry 5070 Paper 42, May/June 2025

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5070/42 · May/June 2025

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A student investigates how the solubility of solid potassium nitrate varies with temperature. The student: step 1 puts $100 {cm}^{3}$ of water into a beaker at room temperature step 2 measures the mass of the beaker of water step 3 adds some potassium nitrate to the water step 4 stirs the mixture until all the potassium nitrate dissolves step 5 repeats steps 3 and 4 until a small amount of undissolved solid remains in the beaker step 6 measures the mass of the beaker of water and potassium nitrate step 7 repeats steps 1 to 6 using water at different temperatures step 8 calculates the mass of potassium nitrate dissolved in the water at each temperature.

a(i)

The student does not make an important measurement. State the measurement that the student needs to make.

[1]

a(ii)

The student’s results are shown in Table 1.1. Table 1.1

temperature $/^{\circ} C$	mass of potassium nitrate dissolved in $100 {cm}^{3}$ of water $/ g$
15	26.4
$room temperature$	31.7
35	54.9
55
75	152.4
95	225.3

Estimate the mass of potassium nitrate dissolved in $100 {cm}^{3}$ of water at $55^{\circ} C$ . mass .......................................................g

[1]

a(iii)

The student observes that when the solution made at $75^{\circ} C$ is left at room temperature a solid appears in the beaker. Explain why.

[2]

Suggest why the student does not do the experiment at $115^{\circ} C$ .

[1]

Explain why the method the student uses gives a higher value for the solubility than the true value.

[1]

Solid $W$ is an impure organic acid, $H_{3} A$ . A student determines the mass of $H_{3} A$ in a sample of $W$ by titration with $0.100 {mol/dm}^{3}$ sodium hydroxide, $NaOH$ . The student: step 1 places the sample of $W$ in a beaker step 2 uses a measuring cylinder to add $100 {cm}^{3}$ of distilled water to the beaker step 3 stirs the mixture until the solid is fully dissolved step 4 labels this mixture $X$ step 5 pipettes $25.0 {cm}^{3}$ of $NaOH$ into a conical flask and adds five drops of thymolphthalein indicator step 6 fills the burette with $X$ step 7 adds $X$ to the conical flask until the solution just changes colour step 8 repeats this titration two more times.

Explain why it is important to use distilled water and not tap water for the experiment.

[1]

b(i)

Fig. 2.1 shows the initial and final burette readings for titration 1. Table 2.1 shows some of the student’s results. Complete Table 2.1 by: • writing the initial and final readings for titration 1 • calculating the volume of $X$ used in each titration • ticking (✓) the best titration results. Table 2.1

	$titration 1$	$titration 2$	$titration 3$
$final burette reading / {cm}^{3}$		19.2	38.6
$initial burette reading / {cm}^{3}$		0.1	19.3
$volume used / {cm}^{3}$
$best titration results (✓)$

[3]

b(ii)

Use the best titration results (✓) to calculate the average volume of $X$ used. average volume .................................................. ${cm}^{3}$

[1]

Calculate the number of moles of $NaOH$ in $25 {cm}^{3}$ of $0.100 {mol/dm}^{3}$ $NaOH$ . number of moles .........................................................

[1]

The equation for the reaction between $H_{3} A$ and $NaOH$ is shown. $H_{3} A + 3 NaOH \to {Na}_{3} A + 3 H_{2} O$ Calculate the number of moles of $H_{3} A$ that react with $25.0 {cm}^{3}$ of $0.100 {mol/dm}^{3}$ $NaOH$ . number of moles .........................................................

[1]

Calculate the number of moles of $H_{3} A$ in the sample of $W$ . number of moles .........................................................

[1]

The relative molecular mass of $H_{3} A$ is 210. Calculate the mass of $H_{3} A$ in the sample of $W$ . mass .......................................................g

[1]

In step 2 a measuring cylinder is used to add $100 {cm}^{3}$ of distilled water to the beaker. Explain why using the measuring cylinder makes the volume of distilled water used inaccurate. Suggest an improvement. explanation ............................................................................................................................... ................................................................................................................................................... improvement .............................................................................................................................

[2]

h(i)

Before the student does step 6 the burette is rinsed with distilled water and then with $X$ . Explain why the burette is rinsed with distilled water.

[1]

h(ii)

Suggest and explain the effect on the titration results if the burette is not rinsed with $X$ after rinsing with distilled water. effect .................................................................................................................................. explanation ........................................................................................................................ ...........................................................................................................................................

[2]

A student investigates solid $A$ and aqueous solution $B$ . Solid $A$ is a metal carbonate. Solution $B$ contains only one cation and one anion. The student does a flame test on solid $A$ .

a(i)

Describe how to do a flame test on a solid.

[2]

a(ii)

The student observes a blue-green colour in the flame. Identify the cation present in $A$ .

[1]

b(i)

The student adds dilute hydrochloric acid to $A$ . An aqueous solution $C$ and a colourless gas are formed. Describe the observations the student makes. Describe the chemical test and the positive result used to identify the gas formed. observations ...................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... test .................................................................................................................................... result .................................................................................................................................

[5]

b(ii)

Explain how the student knows when the reaction has finished.

[1]

Explain how the student uses aqueous ammonia to confirm the metal ion present in $C$ . Include the observations you expect.

[4]

The student does tests on $B$ . The results are shown in Table 3.1. Table 3.1

test	$observation$
adds dilute nitric acid and aqueous barium nitrate	$colourless solution formed$
adds dilute nitric acid and aqueous silver nitrate	$cream precipitate formed$
adds aqueous sodium hydroxide and aluminium foil then warms the mixture	$colourless solution formed$

Identify the anion in $B$ .

[1]

$Q$ is a mixture of solid copper(II) carbonate and solid lead sulfate. Lead sulfate is insoluble in water and does not react with dilute sulfuric acid. Copper(II) carbonate is insoluble in water. It reacts with dilute sulfuric acid to form copper(II) sulfate solution. Plan an investigation to obtain pure copper(II) sulfate crystals and pure lead sulfate solid from $Q$ . Your plan should describe the use of common laboratory apparatus, dilute sulfuric acid and $Q$ . No other chemicals should be used. Your plan should include: • the apparatus needed • the preparation of copper(II) sulfate solution • the method to obtain pure copper(II) sulfate crystals • the method to obtain pure lead sulfate solid • how to test that the lead sulfate is pure. You may draw a diagram to help answer the question.

[6]

Mark scheme

5070/42 · May/June 2025

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1a(i) · 1 mark

1 mark
the temperature

1a(ii) · 1 mark

1 mark
80 – 120 g

1a(iii) · 2 marks

M1
the solution cooled down.
M2
solubility of potassium nitrate decreases (at lower temperatures) / potassium nitrate is less soluble (at lower temperatures)

1b · 1 mark

1 mark
water boils (at 100 ºC)

1c · 1 mark

1 mark
some of the solid is not dissolved

2a · 1 mark

1 mark
tap water is impure / contains dissolved substances / minerals

2b(i) · 3 marks

M1
final reading 23.8 and initial reading 4.2
M2
19.6:19.1:19.3 (all three values to 1dp)
M3
ticks 19.1 and 19.3

2b(ii) · 1 mark

1 mark
average = 19.2.

2c · 1 mark

1 mark
0.0025 / 2.5  10-3

2d · 1 mark

1 mark
0.000833 / 8.33  10-4

2e · 1 mark

1 mark
0.00434 / 4.34  10-3

2f · 1 mark

1 mark
0.91

2g · 2 marks

M1
measuring cylinder does not have high resolution / does not read to 1dp / only reads to 0.5 cm3
M2
use burette / pipette.

2h(i) · 1 mark

1 mark
(washed with distilled water to) remove impurities (from the burette)

2h(ii) · 2 marks

M1
Titration result increases
M2
Because water will dilute the X / concentration of X decreases

3a(i) · 2 marks

M1
dip wire / splint / rod into solid
M2
Put wire / splint into blue / roaring / airhole open / non-luminous / colourless flame

3a(ii) · 1 mark

1 mark
copper / Cu2+

3b(i) · 5 marks

M1
effervescence / fizzing / bubbling.
M2
solid dissolves / disappears
M3
a coloured / blue / green solution is formed
M4
(test gas with) limewater
M5
(limewater turns) milky

3b(ii) · 1 mark

1 mark
no solid remains undissolved / effervescence stops

3c · 4 marks

M1
add ammonia
M2
(pale) blue ppt.
M3
(precipitate) soluble in excess
M4
dark blue solution

3d · 1 mark

1 mark
bromide / Br– ions

4 · 6 marks

M1
apparatus ✓1: filter funnel AND paper AND suitable container (e.g. conical flask/beaker)
M2
apparatus ✓1: Bunsen burner / water bath / electric heater / melting point apparatus
M3
prepare copper(II) sulfate ✓2: add sulfuric acid to the mixture / Q
M4
prepare copper(II) sulfate ✓2: use excess acid / add acid until effervescence stops
M5
obtain copper(II) sulfate crystals ✓3: heat / evaporate the filtrate until saturated / crystals start to form / until half the water evaporated
M6
obtain copper(II) sulfate crystals ✓3: leave to cool / crystalise
M7
obtain lead sulfate solid ✓4: filter (after adding acid to Q)
M8
obtain lead sulfate solid ✓4: wash and dry residue (after filtration of copper(II) sulfate)
M9
test lead sulfate purity ✓5: measure (and compare) the melting point (of residue).

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