3.1 Physical Chemistry Core Practicals

1a2 marks

A student used a precipitation titration to determine the value of x in the formula of a sample of hydrated barium chloride, BaCl₂ ∙xH₂O.

Procedure

Step 1	Prepare a solution by dissolving 1.57 g of BaCl₂ ·xH₂O in deionised water, making the solution up to the mark in a 250.0 cm³ volumetric flask and then mixing thoroughly.
Step 2	Use a pipette to transfer 10.0 cm³of the barium chloride solution into a conical flask. Add excess sodium sulfate solution and swirl the mixture.
Step 3	Fill a burette with 0.0324 mol dm^–3 silver nitrate solution.
Step 4	Add three drops of potassium chromate(VI) solution to the conical flask and titrate the contents, while swirling, with the silver nitrate solution. The end-point is shown by the appearance of a permanent pale red precipitate.
Step 5	Repeat Steps 2 to 4 until concordant results are obtained.

During the titration, two precipitation reactions occur.

Reaction 1

Silver ions react with chloride ions forming silver chloride.

Ag⁺(aq) + Cl^–(aq) → AgCl (s)

Reaction 2

Once all chloride ions have reacted, silver ions react with chromate(VI) ions to form a red precipitate of silver chromate(VI).

2Ag⁺(aq) + Cr $O_{4}^{2 -}$ (aq) → Ag₂CrO₄(s)

Give the ionic equation for the reaction that occurs when sodium sulfate solution is added to the conical flask in Step 2.
Include state symbols.

(1)

ii)

Give a possible reason why it is necessary to add sodium sulfate solution.
Justify your answer.

(1)

How did you do?

1 mark

Suggest why the red precipitate of silver chromate(VI) only forms after all the chloride ions have reacted.

How did you do?

7 marks

Some data obtained in the experiment are shown.

Titration number	1	2	3	4
Burette reading (final) / cm³	16.15	32.05	48.30	47.40
Burette reading (initial) / cm³	0.00	16.15	32.50	31.55
Titre / cm³	16.15

Complete the table and use the concordant results to calculate the mean titre.

(2)

ii)

Determine the value of x in the formula of the hydrated salt, BaCl₂ ·xH₂O.
Use information from the procedure and your mean titre from (c)(i).
You must show your working.

(5)

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1a3 marks

This question is about ammonium chloride, NH₄Cl, a soluble ionic compound.

An aqueous solution of NH₄Cl contains both ammonium ions, N $H_{4}^{+}$ , and chloride ions, Cl^–.

State what would be seen on the addition of acidified silver nitrate solution to an aqueous solution of NH₄Cl.

(1)

ii)

Describe a test to confirm the presence of N $H_{4}^{+}$ ions in a solution of NH₄Cl.
Include the result of the positive test.

(2)

How did you do?

7 marks

A student investigated the enthalpy change when dissolving NH₄Cl in excess water.

NH₄Cl (s) + aq → N $H_{4}^{+}$ (aq) + Cl⁻(aq)

Procedure

Step 1	Accurately weigh 7.17 g of NH₄Cl into a glass beaker.
Step 2	Fill a 50 cm³ measuring cylinder with deionised water. Measure the temperature of the water using a thermometer.
Step 3	Pour the water from the measuring cylinder into the beaker and at the same time start a stopwatch. Stir the solution in the beaker, using the thermometer.
Step 4	Record the temperature at 15 s, 30 s and then at 30 s intervals while continuing to stir the solution.

The data from the experiment are shown on the graph.

q1b-paper-3-jan-2022-edexcel-ial-chemistry

Give two reasons why the student stirred the solution in Steps 3 and 4.

(2)

ii)

Use the graph to determine the maximum temperature change, ΔT, in this experiment. You must show your working on the graph.

(2)

iii)

Another student carried out the experiment using a polystyrene cup in place of the glass beaker.

Explain how this student’s graph would be different.
You may annotate the graph as part of your answer.

(3)

How did you do?

4 marks

The experimental results of another student were used in the equation shown to calculate the enthalpy change, ΔH, for dissolving one mole of NH₄Cl in excess water.

ΔH = $\frac{m \times c \times ∆ T}{n}$
= +14500 J mol^–1

In the equation
m = mass of solution = 50 g
c = specific heat capacity of water = 4.18 J g^–1 °C^–1
ΔT = maximum temperature change of solution
n = moles of NH₄Cl

State two assumptions made in this calculation.
You do not need to justify your answers.

(2)

ii)

The total percentage uncertainty in this experiment was 2.6%.
Show that the enthalpy change of 14.5 kJ mol^–1 is consistent with a data book value of 14.8 kJ mol^–1

(2)

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3 marks

Sodium hydroxide solution reacts with carbon dioxide in the air and should be standardised before use. Ethanedioic acid may be used for this standardisation.

A standard solution of ethanedioic acid, (COOH)₂ , is prepared.

2.40 g of solid ethanedioic acid is dissolved in approximately 100 cm³of deionised water in a beaker.
The solution is transferred into a 250.0 cm³volumetric flask and made up to the mark with deionised water.

Give a possible reason why any solution remaining in the beaker is washed into the volumetric flask before making up to the mark.

(1)

ii)

Calculate the concentration of this standard solution of ethanedioic acid in mol dm^–3.

Give your answer to an appropriate number of significant figures.

[Molar mass of ethanedioic acid = 90.0 g mol⁻¹]

(2)

How did you do?

2b7 marks

A different standard solution of ethanedioic acid is used to determine the concentration of a sodium hydroxide solution J.

Procedure

Step 1	A burette is rinsed with deionised water.
Step 2	The burette is then rinsed with 0.0900 mol dm^–3 ethanedioic acid and filled with this acid solution.
Step 3	A pipette is used to transfer 25.0 cm³ portions of solution J to conical flasks.
Step 4	The portions are titrated with the ethanedioic acid solution using phenolphthalein indicator.

Explain why the burette is rinsed with ethanedioic acid solution in Step 2.

(1)

ii)

The diagram shows how the student read the filled pipette in Step 3.

Identify the two mistakes the student made.

(2)

iii)

The student completely emptied the pipette for each transfer in Step 3.

Explain the effect on the titre of completely emptying the pipette rather than leaving a small amount of solution in the tip.

(2)

iv)

State the colour change in the conical flask at the end-point.

(2)

From ..............................................to .............................................

How did you do?

5 marks

The titration results are shown.

Titration	1	2	3
Final reading / cm³	25.05	26.60	25.50
Initial reading / cm³	0.00	2.00	1.00
Titre / cm³
Titres used in calculation of mean

Complete the table and calculate the mean titre.

(2)

ii)

Calculate the concentration of the sodium hydroxide solution in mol dm^–3.
The equation for the titration is

(COOH)₂+ 2NaOH → (COONa)₂ + 2H₂O

(3)

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4a2 marks

A saturated solution of barium hydroxide was formed by adding barium oxide to water until no more would dissolve. The equation for the reaction is

BaO (s) + H₂O (l) → Ba(OH)₂(aq)

The resulting mixture was filtered to remove excess solid.

The concentration of the barium hydroxide solution was found by titrating portions of the saturated solution with hydrochloric acid of known concentration.

10.0 cm³ portions of the saturated barium hydroxide solution were placed in conical flasks and titrated with 0.200 mol dm⁻³ hydrochloric acid added from a burette.

Three drops of methyl orange indicator were added to the solution in each conical flask.

State the colour change observed at the end-point of the titration.

From .................................................... to ....................................................

How did you do?

7 marks

Some of the results are shown.

Titration	1	2	3	4
Final burette reading/ cm³	22.60	44.45	23.05
Initial burette reading/ cm³	0.10	22.60	1.25	23.20
Titre/ cm³	22.50	21.85		21.90

Complete the table.

(1)

ii)

Give a reason why the first titre should not be used to calculate the mean titre.

(1)

iii)

Calculate the number of moles of hydrochloric acid in the mean titre.

(2)

iv)

The equation for the reaction in the titration is

Ba(OH)₂(aq) + 2HCl(aq) → BaCl₂(aq) + 2H₂O(l)

Calculate the concentration of barium hydroxide, in g dm⁻³, giving your answer to an appropriate number of significant figures.

(3)

How did you do?

4c1 mark

Solid samples of soluble barium compounds such as barium oxide are toxic by inhalation due to the presence of barium ions.

Give a safety precaution that should be used to minimise this risk when adding barium oxide to water.

How did you do?

4d1 mark

Barium also forms a peroxide. A bottle of barium peroxide has the hazard symbol .
q2d-paper-3-2021-june-edexcel-ial-chemistry Give the meaning of this symbol.

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	Download	View
Medium
Hard

Time/minutes	0	1	2	2.5	3	4	5	6	7	8
Temperature/°C	21.1	21.0	21.0	X	34.2	37.6	36.9	36.1	35.2	34.3

Measurement	Value
Volume of carbon dioxide / cm³	95
Mass of test tube + carbonate / g	21.69
Mass of test tube / g	21.36
Mass of carbonate / g	0.33

		Titration number
	Rough	1	2	3	4
Final reading / cm³	24.90	21.25	42.85	21.80	43.15
Initial reading / cm³	2.30	0.00	21.25	0.50	21.80
Titre/ cm³					21.35

Edexcel International A Level Chemistry

Topic Questions

3.1 Physical Chemistry Core Practicals