A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Gas Laws and Ideal Gas Equation — mark scheme explained
The short answer
The study of gas laws is fundamental to understanding the behavior of gases under various conditions. These laws describe how pressure (p), volume (V), temperature (T), and mass of a gas are related.
The question
A sealed container with a volume of 2 m 3 contains an ideal gas at a temperature of 300 K. The pressure inside the container is 100,000 Pa. Calculate the number of moles of gas in the container.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculation questions, show all steps clearly and include units in your final answer. For conceptual questions, provide clear and concise explanations using key terms and concepts.
What the command words demand
- Calculate
- Perform a numerical calculation to find a specific value.
- Determine
- Find or establish a particular value or quantity.
- Explain
- Provide a detailed account of the reasons for something.
- Describe
- Give a detailed account of the characteristics or features of something.
- Derive
- Show how a formula or equation can be obtained from first principles.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question for this topic to ensure you have enough time to show all working and provide detailed explanations.
- Identify the given values: p = 100,000 Pa, V = 2 m 3 , T = 300 K, R = 8.314 J/(mol·K).1 mark
- Use the ideal gas equation for moles of gas: pV = nRT.1 mark
- Rearrange to solve for n: n = (pV) / (RT).1 mark
- Substitute the values: n = (100,000 × 2) / (8.314 × 300).1 mark
- Calculate: n ≈ 80.6 moles.1 mark
Final answer: 80.6 moles
Work through every step correctly and you earn all 5 marks.
Another worked example
A gas at a constant temperature of 273 K has an initial volume of 1 m 3 and pressure of 100,000 Pa. If the volume is reduced to 0.5 m 3 , what will be the new pressure?
- Identify the given values: p 1 = 100,000 Pa, V 1 = 1 m 3 , V 2 = 0.5 m 3 , T is constant.0 marks
- Use Boyle's Law: p 1 V 1 = p 2 V 2 .1 mark
- Rearrange to solve for p 2 : p 2 = (p 1 V 1 ) / V 2 .1 mark
- Substitute the values: p 2 = (100,000 × 1) / 0.5.1 mark
- Calculate: p 2 = 200,000 Pa.1 mark
Final answer: 200,000 Pa
Work through every step correctly and you earn all 4 marks.
Common mistakes
Confusing the units of pressure and volume in the ideal gas equation.
Why it happens: Students often mix up the units for pressure (Pa) and volume (m 3 ), leading to incorrect calculations.
Fix: Always double-check the units before substituting values into the ideal gas equation. Ensure pressure is in Pascals (Pa) and volume is in cubic meters (m 3 ).
Using absolute temperature in Celsius instead of Kelvin.
Why it happens: Students sometimes forget to convert temperatures from Celsius to Kelvin, which can lead to significant errors in calculations involving the ideal gas equation.
Fix: Always use absolute temperature (Kelvin) in the ideal gas equation. Convert Celsius to Kelvin by adding 273.15.
Forgetting to convert volume from liters to cubic meters.
Why it happens: Students often use volume in liters without converting it to cubic meters, which can result in incorrect answers.
Fix: Convert volume from liters to cubic meters by dividing by 1000. For example, 1 liter = 0.001 m 3 .
Misapplying Boyle's Law or Charles's Law in practical experiments.
Why it happens: Students may incorrectly apply the conditions for Boyle's Law (constant temperature) and Charles's Law (constant pressure), leading to incorrect experimental results.
Fix: Ensure that the correct conditions are maintained during the experiment. For Boyle's Law, keep the temperature constant; for Charles's Law, keep the pressure constant.
Confusing molar mass with molecular mass.
Why it happens: Students sometimes mix up molar mass (g/mol) and molecular mass (amu), leading to confusion in calculations involving gas properties.
Fix: Understand the difference between molar mass and molecular mass. Molar mass is the mass of one mole of a substance, while molecular mass is the mass of one molecule.
Incorrectly calculating work done by a gas.
Why it happens: Students may use the wrong formula or sign for work done, leading to incorrect results.
Fix: Use the correct formula W = pΔV and pay attention to the sign of ΔV. If the volume decreases (compression), ΔV is negative; if the volume increases (expansion), ΔV is positive.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Ideal Gas Calculation | Use the ideal gas equation to find the number of moles from p, V and T. | 5 |
| Apply Boyle's Law | Use Boyle's Law to find the new pressure after the gas volume is reduced. | 4 |
| Charles's Law Calculation | Find the new volume of a gas after its temperature changes at constant pressure. | 4 |
| Calculate Work Done | Use W = pΔV to find the work done when a gas is compressed at constant pressure. | 4 |
| Total across these question types | 17 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.