AQA A-Level Chemistry AS Paper 1, November 2021: Question 10

9 marks · Medium difficulty · State/Explain/Numerical

Calculate the equilibrium constant expression, moles of oxygen at equilibrium, and temperature using the ideal gas equation for the oxidation of sulfur dioxide.

Practise this question

Question

A three-part chemistry question about the reaction 2SO2(g) + O2(g) <=> 2SO3(g). Part 10.1 asks for the Kc expression for the reaction (1 mark). Part 10.2 gives equilibrium amounts in an 1800 cm3 container and asks to calculate the amount of oxygen in moles at equilibrium (3 marks). Part 10.3 gives a mixture of gases in a 3500 cm3 vessel at 255 kPa and asks to calculate the temperature in degrees Celsius (5 marks).
Question text

10 Sulfur dioxide reacts with oxygen to form sulfur trioxide.

2 SO (g) + O (g) ⇌ 2 SO (g) ΔH = –196 kJ mol−1

22 3

10.1 Give an expression for the equilibrium constant (Kc) for this reaction.

[1 mark]

Kc

10.2 A mixture of sulfur dioxide and oxygen is allowed to reach equilibrium in a container of

volume 1800 cm3 at temperature T.

At equilibrium, the mixture contains 0.176 mol of sulfur dioxide and

0.461 mol of sulfur trioxide.

At temperature T the equilibrium constant, K = 15.0 mol–1 dm3

c

Calculate the amount, in moles, of oxygen at equilibrium.

[3 marks]

Amount of oxygen mol

10.3 At a different temperature, a mixture contains

0.025 mol of sulfur dioxide

0.049 mol of oxygen

0.034 mol of sulfur trioxide.

The total pressure of the mixture in a 3500 cm3 reaction vessel is 255 kPa

Use the data to calculate the temperature, in °C, of the mixture.

The ideal gas constant, R = 8.31 J K−1 mol−1

[5 marks]

Temperature °C

Section B

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Mark scheme

Show the mark scheme Mark scheme showing answers for questions 10.1, 10.2, and 10.3. Part 10.1 shows Kc = [SO3]2 / ([SO2]2[O2]). Part 10.2 outlines dividing by volume, substituting into rearranged expression to find concentration of oxygen, and multiplying by volume to get 0.823 mol. Part 10.3 outlines total moles calculation, unit conversions, rearrangement of PV=nRT, and conversion of Kelvin to Celsius resulting in 721 °C.

Question Marking guidance Additional Comments/Guidelines Mark

[SO ]2 1

10.1 𝐾𝑐 = 2

[SO2] [O2]

M1: dividing by volume for SO2 and SO3 / calculation of 1

concentrations of SO2 and SO3

0.461 2

( )

15.0 = 1.80

0.176 2

( ) [𝑂2]

1.80

Or [SO ] = 0.0978 mol dm–3 and [SO ] = 0.256 mol dm–3

M2: correct substitution into rearranged expression

0.461 2

10.2 ( )

[𝑂 ] = 1.80

0.176

( ) (15.0)

1.80

or

(0.256)2

[𝑂2] = 2

(0.0978) (15.0)

([O ] = 0.457 mol dm–3)

M3 amount of oxygen = [O2] × 1.80 = 0.823 mol At least 2sf – MISTRY – – JUNE 20211

(pV = nRT) 1

T = pV ÷ nR M1: rearranged expression for ideal gas equation

n = 0.025 + 0.049 + 0.034

n = 0.108 M2: total number of moles 1

Conversions: pressure = 255000 Pa ; volume = 0.0035 m3 M3: unit conversions 1

10.3

255000 × 0.0035

T =

8.31 × 0.108

T = 994.5 K M4: temperature in K 1

T = 721 °C M5: temperature in °C (allow 720 – 722) 1

M5 = M4 – 273

How to answer it

Equilibrium Constants & Ideal Gas Calculations

What this question tests

This multi-part physical chemistry question assesses your ability to write expressions for the equilibrium constant ( Kc ), manipulate algebraic equations to find equilibrium concentrations and moles, use the ideal gas equation ( pV = nRT ), and correctly handle unit conversions (particularly volume from cm³ to m³ and temperature from Kelvin to degrees Celsius).

Part 10.1

Expression for the Equilibrium Constant (Kc)

✅ Correct Answer

Kc = [SO3]² / ([SO2]² [O2])

1 mark

💡 Key Knowledge

  • Products go on the numerator (top); reactants go on the denominator (bottom).
  • The stoichiometric balancing numbers from the equation become the powers.
  • Square brackets denote equilibrium concentrations in mol dm⁻³ .
Part 10.2

Calculating Equilibrium Moles of Oxygen

✅ Correct Answer

Amount of oxygen = 0.823 mol (Accept 2 significant figures or more)

3 marks total

🧠 Exam Technique & Step-by-Step Calculation

  1. Step 1 (M1): Convert moles to concentration by dividing by the volume in dm³. Volume = 1800 cm³ = 1.80 dm³. Alternatively, substitute moles directly over the volume term 1.80 inside the Kc expression.
  2. Step 2 (M2): Rearrange the Kc expression to make [O2] the subject and substitute values:
    [O2] = (0.461 / 1.80)² / [ (0.176 / 1.80)² × 15.0 ] = 0.457 mol dm⁻³ .
  3. Step 3 (M3): Convert concentration back to moles by multiplying by the volume in dm³ ( 0.457 × 1.80 = 0.823 mol ).

❌ Common Errors & Examiner Commentary

  • Volume omission: Forgetting to divide moles by 1.80 dm³ to find concentrations before putting them into Kc .
  • Rearrangement errors: Incorrectly algebra-flipping squared terms, leading to inverted calculations. Note that volume terms often cancel out depending on how you write the ratio, but setting up concentrations cleanly avoids this trap.
Part 10.3

Calculating Temperature using the Ideal Gas Equation

✅ Correct Answer

Temperature = 721 °C (Accept range 720 – 722 °C )

5 marks total

📐 Step-by-Step Calculation

  1. Step 1 (M1): Rearrange pV = nRT to find temperature: T = pV / (n × R)
  2. Step 2 (M2): Find total moles ( n ) by summing all equilibrium species:
    n = 0.025 + 0.049 + 0.034 = 0.108 mol
  3. Step 3 (M3): Perform vital unit conversions:
    • Pressure ( p ): 255 kPa = 255 000 Pa (multiply by 1000)
    • Volume ( V ): 3500 cm³ = 0.0035 m³ (divide by 1 000 000)
  4. Step 4 (M4): Calculate temperature in Kelvin:
    T = (255 000 × 0.0035) / (0.108 × 8.31) = 994.5 K
  5. Step 5 (M5): Convert Kelvin to Celsius by subtracting 273:
    994.5 - 273 = 721.5 °C (rounds to 721 °C )

❌ Common Errors & Examiner Commentary

  • Unit Conversion Failures: This is the #1 place students lose marks. Failing to convert cm³ to m³ (using 3500 instead of 0.0035 ) or failing to convert kPa to Pa ruins the entire calculation.
  • Forgetting to subtract 273: Leaving the final temperature value in Kelvin ( 994.5 K ) when the question explicitly requested the answer in °C .
  • Incomplete total moles: Forgetting to add all three gas components together to find total n .

Topics

Physical Chemistry · 3.1.2 Amount of Substance · 3.1.6 Chemical Equilibria, Le Chatelier's Principle and Kc

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.