A-Level · Chemistry · AQA · Mark scheme decoded

AQA A-Level Chemistry: The Ideal Gas Equation — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Chemistry specificationlast verified 3 July 2026

The short answer

The ideal gas equation, pV = nRT, is a fundamental concept in physical chemistry that describes the behavior of gases under various conditions. This equation relates four key variables: pressure (p), volume (V), amount of substance (n), and temperature (T).

The question

A sample of an ideal gas has a volume of 0.5 m 3 , contains 2 moles, and is at a temperature of 300 K. Calculate the pressure of the gas.

[Paraphrased for study — not reproduced from any exam paper.]

4 marks

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, marks are typically awarded for correct substitution of values into formulas, accurate arithmetic, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic.

What the command words demand

Calculate: Perform a numerical calculation using given data and appropriate formulas.
Determine: Find or derive a specific value or quantity based on given information.
Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Analyze: Examine and interpret data or graphs to draw conclusions about the behavior of the system.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, perform calculations, and check your work.

Identify the given values.0 marks
V = 0.5 m 3n = 2 molT = 300 KR = 8.314 J/(mol·K)
Use the ideal gas equation to find pressure (p).1 mark
pV = nRTp = nRT / V
Substitute the values into the formula.1 mark
p = (2 mol × 8.314 J/(mol·K) × 300 K) / 0.5 m 3
Perform the multiplication and division.2 marks
p = (4988.4 J) / 0.5 m 3p = 9976.8 Pa

Final answer: 9976.8 Pa

Work through every step correctly and you earn all 4 marks.

Another worked example

A gas is contained in a cylinder with a volume of 2 L at a pressure of 1 atm and a temperature of 273 K. Calculate the number of moles of gas.

4 marks

Identify the given values.1 mark
V = 2 L = 0.002 m 3p = 1 atm = 101325 PaT = 273 KR = 8.314 J/(mol·K)
Use the ideal gas equation to find the number of moles (n).1 mark
pV = nRTn = pV / RT
Substitute the values into the formula.1 mark
n = (101325 Pa × 0.002 m 3 ) / (8.314 J/(mol·K) × 273 K)
Perform the multiplication and division.1 mark
n = 202.65 J / 2271.642 J/moln ≈ 0.09 moles

Final answer: 0.09 moles

Work through every step correctly and you earn all 4 marks.

Common mistakes

Forgetting to convert units before using them in the ideal gas equation.
Why it happens: Students often overlook the need to ensure all variables are in their correct SI units, leading to incorrect calculations.
Fix: Always check and convert units to pascals (Pa) for pressure, cubic meters (m 3 ) for volume, moles (mol) for amount of substance, and kelvins (K) for temperature before substituting them into the equation.
Using the wrong value for the gas constant R.
Why it happens: Students may use an incorrect value for R or forget to convert it to the appropriate units, leading to errors in calculations.
Fix: Always use the correct value of R (8.314 J/(mol·K) or 0.0821 L·atm/(mol·K)) and ensure it matches the units of the other variables.
Incorrectly rearranging the ideal gas equation.
Why it happens: Students may make algebraic errors when rearranging the equation to solve for a specific variable, leading to incorrect answers.
Fix: Practice rearranging the equation step-by-step and double-check your work to ensure accuracy.
Misinterpreting the relationship between variables in the ideal gas equation.
Why it happens: Students may not fully understand how changes in one variable affect another, leading to incorrect conclusions.
Fix: Practice problems that involve changing one variable and observing the effect on others. Understand the direct and inverse relationships between pressure, volume, amount of substance, and temperature.
Using the wrong formula for conversions.
Why it happens: Students may use incorrect conversion factors when converting units, leading to errors in calculations.
Fix: Memorize common conversion factors (e.g., 1 L = 0.001 m 3 , 1 atm = 101325 Pa) and double-check your conversions.
Forgetting to use absolute temperature (K).
Why it happens: Students may use Celsius or Fahrenheit instead of kelvins, leading to incorrect calculations.
Fix: Always convert temperatures to kelvins by adding 273.15 to the Celsius temperature before using them in the ideal gas equation.

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 Law	Rearrange the ideal gas equation and calculate the pressure of a gas sample.	4
Ideal Gas Calculation	Use the ideal gas equation to find moles from pressure, volume and temperature.	4
Ideal Gas Calculation	Use the ideal gas equation to find the volume from pressure, moles, and temperature.	4
Ideal Gas Law	Rearrange pV = nRT to find temperature, substituting given values with correct SI units.	4
Total across these question types		16

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.