O Level Chemistry 5070 Paper 41, May/June 2023

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5070/41 · May/June 2023

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A student uses chromatography to separate the dyes in a food colouring. Fig. 1.1 shows the apparatus the student uses.

Name apparatus X.

[1]

State why it is important to have a lid on X.

[1]

State why the start line is: • above the level of the solvent • drawn in pencil. above the level of the solvent drawn in pencil

[2]

The $R_{f}$ value of one of the dyes in the food colouring is 0.60. In the separation this dye travels $5.7 c m$ . Calculate the distance travelled by the solvent during the separation.

[2]

White vinegar is a colourless solution containing ethanoic acid. A student titrates $0.400 m o l / d m^{3}$ aqueous sodium hydroxide with two samples of vinegar, A and B. The student does four titrations, two with A and two with B. The student: Step 1. uses a volumetric pipette to add $25.0 c m^{3}$ of $0.400 m o l / d m^{3}$ sodium hydroxide to a conical flask Step 2. adds five drops of an indicator to the conical flask Step 3. adds A from a burette while swirling the contents of the flask, adding drop by drop near the end-point, until the solution just changes colour Step 4. empties the conical flask and rinses it with distilled water Step 5. repeats steps 1 to 4 Step 6. repeats steps 1 to 5 using B instead of A.

a(i)

Fig. 2.1 shows the burette readings for the two titrations with vinegar A. Record the burette readings in Table 2.1. Complete Table 2.1 with the volume used in each titration.

[3]

a(ii)

Fig. 2.2 shows the burette readings for one of the titrations with vinegar B. Record the burette readings for Titration 4 in Table 2.2. Calculate the volume of vinegar B added in Titrations 3 and 4.

[1]

Calculate the mean volume of A and of B needed to neutralise $25.0 c m^{3}$ of $0.400 m o l / d m^{3}$ aqueous sodium hydroxide.

[1]

Suggest why the titrations using A and B are repeated.

[1]

The equation for the reaction between ethanoic acid and sodium hydroxide is shown. $C H_{3} C O O H + N a O H \to C H_{3} C O O N a + H_{2} O$ The answer to (b) shows the mean volume of B used to neutralise $25.0 c m^{3}$ of $0.400 m o l / d m^{3}$ aqueous sodium hydroxide. Calculate the concentration of ethanoic acid in B. Give your answer to an appropriate number of significant figures.

[2]

Use your answer to (d) to calculate the mass of ethanoic acid in $500 c m^{3}$ of B. [ $M_{r}$ : ethanoic acid, 60]

[2]

Use your answers to (b) and (e) to calculate the mass of ethanoic acid in $500 c m^{3}$ of A.

[1]

Suggest why the conical flask is rinsed with distilled water and not $0.400 m o l / d m^{3}$ aqueous sodium hydroxide between titrations.

[2]

Some vinegars are brown rather than colourless. Suggest why this titration method would not be suitable for finding the concentration of ethanoic acid in brown vinegar.

[1]

A student investigates solution W and copper(II) carbonate.

a(i)

The tests the student does on W are shown in Table 3.1. Some of the observations for these tests are also shown. The student correctly concludes that ammonia is formed in test 1. Describe what else the student must have done to reach this conclusion.

[3]

a(ii)

State why the student cannot identify the anion in W from the observation in test 3.

[1]

a(iii)

State how the tests and observations show that W does not contain sodium ions.

[1]

a(iv)

Identify the cation and anion in W.

[2]

b(i)

The student adds dilute hydrochloric acid to copper(II) carbonate. A gas and a solution are produced. State the observation that confirms the production of a gas.

[1]

b(ii)

Describe how to do a flame test to confirm the presence of copper(II) ions in the solution.

[3]

b(iii)

Describe one other method the student could use to confirm that the solution contains copper(II) ions.

[3]

Baking powder is used to make cakes rise. When water is added to baking powder, carbon dioxide gas is released. The longer the baking powder is stored, the less carbon dioxide it releases when water is added. Plan an investigation to show which of two different samples of baking powder has been stored for longer. Your plan should include the use of common laboratory apparatus, the two samples of baking powder and water. No other chemicals should be used. Your plan should include: • the apparatus needed • the method to use • the measurements to take • the variables to control • how to use the results to determine which sample has been stored for longer. You may draw a diagram to help you answer the question.

[6]

Mark scheme

5070/41 · May/June 2023

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1a · 1 mark

1 mark
beaker

1b · 1 mark

1 mark
prevent evaporation of solvent (from the beaker) OR To ensure the air in the beaker is saturated with vapour

1c · 2 marks

M1
(start line above level of solvent so) dye doesn’t dissolve off paper into solvent
M2
(start line in pencil so) line doesn’t smudge / smear / travel up paper (start line in pencil as) pencil does not dissolve / pencil is insoluble

1d · 2 marks

M1
5.7 / 0.6
M2
9.5 cm

2a(i) · 3 marks

M1
initial and final readings completed correctly
Titration 1 Titration 2
final burette reading /cm³ 24.9 37.4
initial burette reading/ cm³ 0.0 12.3
M2
volume used completed correctly
Titration 1 Titration 2
volume of A used/ cm³ 24.9 25.1
M3
all readings to 1 decimal place

	Titration 1	Titration 2
final burette reading /cm³	24.9	37.4
initial burette reading/ cm³	0.0	12.3

	Titration 1	Titration 2
volume of A used/ cm³	24.9	25.1

2a(ii) · 1 mark

1 mark
initial, final readings and volume used completed correctly
Titration 1 Titration 2
final burette reading / cm³ 25.3
initial burette reading / cm³ 12.6
volume of B used / cm³ 12.5 12.7

	Titration 1	Titration 2
final burette reading / cm³		25.3
initial burette reading / cm³		12.6
volume of B used / cm³	12.5	12.7

2b · 1 mark

1 mark
25.0 and 12.6

2c · 1 mark

1 mark
to check that the results are consistent / reproducible

2d · 2 marks

M1
25 / 1000  0.400 = 0.01(00) moles
M2
0.794 correct to 3 sig. fig.

2e · 2 marks

M1
0.794  0.5
M2
60  M1 = 23.8 g

2f · 1 mark

1 mark
12 g

2g · 2 marks

1 mark
(Distilled water is used to) remove any chemicals from the flask
1 mark
(NaOH is not used because) some NaOH would remain in flask / (NaOH is not used because) more A / B / ethanoic acid would be needed

2h · 1 mark

1 mark
The colour change could not be (clearly) observed

3a(i) · 3 marks

M1
warm
M2
test gas with (damp red) litmus paper
M3
paper turns blue

3a(ii) · 1 mark

1 mark
hydrochloric acid contains chloride / Cl⁻ ions

3a(iii) · 1 mark

1 mark
sodium ions would give a yellow flame

3a(iv) · 2 marks

M1
ammonium / NH₄⁺
M2
sulfate / SO₄²⁻

3b(i) · 1 mark

1 mark
effervescence

3b(ii) · 3 marks

M1
any workable method to introduce solution into flame
M2
blue flame of Bunsen
M3
flame changes colour to blue / green / blue-green

3b(iii) · 3 marks

M1
(aqueous) sodium hydroxide OR (aqueous) ammonia
M2
blue ppt
M3
insoluble in excess OR dark blue solution in excess

4 · 6 marks

1 mark
Apparatus: Balance / Test-tube / conical flask / beaker to hold sample and water / Equipment to measure volume of gas etc. e.g. measuring cylinder with water trough / gas syringe
1 mark
Method and Measurements: Measures a mass / amount of a sample / Adds (excess) water / Reweighs or measures volume of gas produced / counts bubbles / measures time to turn limewater milky
1 mark
Control: Same mass of each sample / amount / volume OR calculate change in mass / volume produced per gram OR same surface area of baking powder / Same temperature / Measure total volume of gas produced when no further change / Measure volume of gas produced in a fixed time / Measure time taken to produce a fixed volume of gas
1 mark
Use of results: Sample that produces more gas / loses more mass (weight) / has more bubbles / has less time to turn milky / is milker is stored for less time or vice versa
1 mark
Any other marking point from (1), (2), (3), or (4)
1 mark
Any other marking point from (1), (2), (3), or (4)

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