Reaction Calculations (Edexcel A Level Chemistry): Exam Questions

The equation for the complete combustion of butane is

2C₄H₁₀ (g) + 13O₂ (g) → 8CO₂ (g) + 10H₂O (l)

What is the minimum volume of oxygen, at room temperature and pressure (r.t.p.), needed for the complete combustion of 0.200 mol of butane? [Molar volume of a gas at r.t.p. = 24.0 dm³ mol⁻¹]

This question is about the molar volume of gases.

i) Calculate the volume of one mole of an ideal gas, A, at 60 °C and 500 kPa pressure.

Give your answer to two significant figures and include units.

[The ideal gas equation is pV = nRT. Gas constant (R) = 8.31 J K⁻¹ mol⁻¹] 

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ii) At room temperature and pressure (r.t.p) another gas B, with formula XH₃ , has a density of 1.42 g dm⁻³. 

Calculate the molar mass of the gas XH₃ and deduce the identity of the element X. 

[The molar volume of gas B = 24000 cm³ mol⁻¹ at r.t.p.]

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The apparatus shown was used to measure the volume of gas evolved when a weighed mass of sodium carbonate reacted with dilute hydrochloric acid.

 The following procedure was used.

1. Solid sodium carbonate was placed in a container and weighed accurately.

2. The delivery tube and rubber bung were removed and the sodium carbonate was transferred to the test tube.

3. The container was then reweighed.

4. The syringe plunger was pushed in, to zero the syringe.

5. 10.0 cm³ of 0.400 mol dm⁻³ hydrochloric acid was then added to the sodium carbonate and the rubber bung and delivery tube rapidly replaced.

6. The mixture was shaken and, when the reaction had finished, the reading of the syringe was noted.

Results 

Mass of container and sodium carbonate before transfer = 20.135 g

 Mass of container after transfer of the sodium carbonate = 19.893 g

 Mass of sodium carbonate used = 0.242 g

 The equation for the reaction is

Na₂CO₃ (s) + 2HCl (aq) → 2NaCl (aq) + CO₂ (g) + H₂O (l)

i) Calculate the moles of hydrochloric acid and the moles of sodium carbonate used in this experiment.

 Use your answers to decide which reactant is in excess.

Calculate the maximum volume of carbon dioxide which could be produced.

 Molar mass of Na₂CO₃ = 106.0 g mol⁻¹

 Molar volume of gas = 24000 cm³ mol⁻¹ at r.t.p.

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 ii) The actual volume of carbon dioxide collected was less than calculated.

Give two reasons for this. 

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The reaction of sulfuric acid with potassium hydroxide is a neutralisation.

The equation for this reaction is

H₂SO₄ (aq) + 2KOH (aq) → K₂SO₄ (aq) + 2H₂O (l)

A titration was carried out using the following method.

1. Potassium hydroxide solution of unknown concentration was placed in a burette and the initial reading was recorded.

2. 25.0 cm³ of sulfuric acid solution, concentration 0.0800 mol dm⁻³, was transferred to a conical flask.

3. Three drops of phenolphthalein indicator were added to the sulfuric acid.

4. Potassium hydroxide was added from the burette until the solution just changed colour and then the burette reading was recorded.

5. Repeat titrations were carried out until concordant titres were obtained.

Select the most appropriate piece of apparatus to measure the 25.0 cm³ of sulfuric acid.

What is the colour of the solution when neutralisation has just occurred? 

i) Complete the table of results for titration number 1, using the diagrams to find the initial and final burette readings.

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Table of results 

ii) The best value for the mean titre of this reaction is 

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iii) Calculate the concentration, in mol dm⁻³, of the potassium hydroxide solution, giving your answer to an appropriate number of significant figures. 

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Many vehicles are fitted with airbags which provide a gas-filled safety cushion to protect the occupant of the vehicle if there is a crash.

The first reaction in airbags is the thermal decomposition of sodium azide, NaN₃, to form sodium and nitrogen gas.

i) Write the equation for this decomposition of sodium azide.

State symbols are not required.

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ii) In the reaction in (a)(i), a typical airbag is inflated by about 67 dm³ of gas. Calculate the minimum mass of sodium azide, in grams, needed to produce this volume of gas. Use the Ideal Gas Equation and give your answer to an appropriate number of significant figures.

For the purpose of this calculation, assume that the temperature is 300 °C and the pressure is 140000 Pa.

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The second reaction in the airbag is between the sodium produced in the reaction (a)(i) and potassium nitrate.

Balance the above equation, justifying your answer in terms of the changes in oxidation numbers.

The third reaction in the airbag is between the metal oxides and silicon dioxide.

State the type of reaction taking place and justify why this reaction is necessary.

The Maxwell-Boltzmann distribution diagram shows the molecular energies for the gaseous system immediately after the airbag has been deployed.

What is the change in shape of the curve when the airbag cools?

This question is about the reaction of magnesium with dilute hydrochloric acid.

Write an equation for the reaction of magnesium with hydrochloric acid.

Include state symbols.

The apparatus shown in the diagram can be used to collect the gas produced during the reaction of magnesium with dilute hydrochloric acid.

The following procedure was used.

i) A measuring cylinder was used to measure the 10.0 cm³ of dilute hydrochloric acid in Step 1. The uncertainty for a volume measurement is ± 0.5 cm³. Calculate the percentage uncertainty in the volume of hydrochloric acid.

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ii) Determine which reactant is in excess by calculating the number of moles of magnesium and of hydrochloric acid used in the experiment.

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iii) Calculate the maximum number of moles of gas that could be produced, using your answers to (a) and (b)(ii).

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iv) Under the conditions of the experiment, the temperature was 23°C and the pressure 98 000 Pa.

Calculate the maximum volume of gas, in cm³, that could be produced using your answer in (b)(iii). Give your answer to an appropriate number of significant figures.

[The ideal gas equation is pV = nRT.  Gas constant (R) = 8.31 J mol^–1 K^–1]   

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i) Deduce two possible reasons why the volume of gas collected in the experiment was smaller than that calculated in (b)(iv).

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ii) Describe two changes to the procedure that would enable the volume of gas collected to be closer to that calculated in (b)(iv).

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This question is about the titration of a weak acid with a strong base.

A standard solution of ethanedioic acid, which is a weak, diprotic acid, can be used to determine the concentration of a sodium hydroxide solution. 25.0 cm³ of the ethanedioic acid solution, with concentration 3.80 g dm⁻³, was pipetted into a conical flask. A few drops of indicator solution were added. The ethanedioic acid was titrated with the sodium hydroxide solution which was in the burette. The titration was repeated and the following results were obtained. 

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

i) In the appropriate row, tick (✔) those titre values that should be used to find the mean, and use these titres to calculate it.

Write the value of the mean titre in the box provided in the table of results.

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ii) Ethanedioic acid is a weak acid. Name a suitable indicator for this titration and state the colour change at the end-point.

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Name of indicator ..................................................

 Colour change at the end-point from
.............................. to ..............................

iii) The equation for the reaction of ethanedioic acid with sodium hydroxide is 

C₂H₂O₄ + 2NaOH → C₂O₄Na₂ + 2H₂O

 Calculate the concentration of the sodium hydroxide solution, in mol dm⁻³.

Give your answer to three significant figures.

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The uncertainty in each burette reading is ± 0.05 cm³. The uncertainty in the pipette volume is ± 0.06 cm³. 

i) Calculate the percentage uncertainties for titre 4, and the pipette volume.

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ii) Which of the following changes would halve the percentage uncertainty in the volume of liquid measured by the burette?

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A group of students analysed a hydrated salt with the formula KH₃(C₂O₄)_y.zH₂O where y and z are whole numbers.

The students carried out experiments to determine the values of y and z.

Experiment 1 – to determine the value of y

One student was provided with a 0.0235 mol dm^-3 solution of the salt.

25.0 cm³ portions of the salt solution were acidified with excess dilute sulfuric acid and heated to about 60 °C.

Each portion was titrated with 0.0203 mol dm^-3 potassium manganate(VII).

The results of four titrations are shown in the table.

i) Complete the diagram to show the final burette reading in Titration 1.

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ii) Explain why this student should use a mean titre of 23.15 cm³ and not 23.43 cm³ in the calculation.

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iii) The uncertainty in each burette reading is ±0.05 cm³.

Calculate the percentage uncertainty in the titre volume of potassium manganate(VII) solution used in Titration 2.

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iv) The equation for the reaction is

2MnO₄^– + 5C₂O₄^2– + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O

Deduce, by calculation, the value of y, to the nearest whole number, in the formula KH₃(C₂O₄)y.zH₂O.

Use the mean titre of 23.15 cm³ and other data from Experiment 1.

You must show your working.

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Experiment 2 – to determine the value of z

Another student wrote an account of the method for this experiment.

(i) Deduce, by calculation, the value of z, to the nearest whole number, in the formula KH₃(C₂O₄)_y.zH₂O.

You must use the data from Experiment 2 and your value of y in (a)(iv).

You must show your working.

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(ii) A third student carried out Experiment 2 and calculated a value of z that was lower than expected.

This student evaluated the experiment and gave two suggestions for z being lower.

Suggestion 1

“Some of the crystals jumped out of the crucible while it was being heated.”

Suggestion 2

“It was difficult to tell when all the water of crystallisation had been lost.”

Evaluate these two suggestions to decide whether they could account for the lower value of z obtained from the experimental results.

Include an explanation of the effect each suggestion would have on the calculated value of z and how the method could be improved to prevent these errors.

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