A LevelPhysicsOCRRevision Notes5. Newtonian World & AstrophysicsIdeal GasesIdeal Gas Equation

Ideal Gas Equation (OCR A Level Physics): Revision Note

Exam code: H556

Written by: Katie M

Reviewed by: Caroline Carroll

Updated on 2 July 2026

Ideal Gas Equation

An ideal gas is a specific type of gas which:
- Has molecules with negligible volume
- Collisions which are elastic
- Cannot be liquified
- Has no interactions between the molecules (except during collisions)
- Obeys the (ideal) gas laws (Boyles law, Charles’ law and Pressure law)
All of these can occur at any temperature or pressure

The ideal gas equation for number of moles can be expressed as:

The ideal gas equation for number of molecules can also be written in the form:

An ideal gas is therefore defined as:
A gas which obeys the equation of state pV = nRT at all pressures, volumes and temperatures

Worked Example

A storage cylinder of an ideal gas has a volume of 8.3 × 10³ cm³. The gas is at a temperature of 15 °C and a pressure of 4.5 × 10⁷ Pa.

Calculate the amount of gas in the cylinder, in moles.

Answer:

Step 1: State the known quantities

Pressure, P = 4.5 × 10⁷ Pa
Volume, V = 8.3 × 10³ cm³ = 8.3 × 10³ × 10⁻⁶ = 8.3 × 10⁻³ m³
Temperature, T = 15 °C = 15 + 273 = 288 K

Step 2: Write down the ideal gas equation

pV = nRT

Step 3: Rearrange for the number of moles n

Step 4: Substitute in values and calculate number of moles of gas

$n = \frac{4.5 \times 10^{7} \times 8.3 \times 10^{- 3}}{8.31 \times 288} = 160 mol$

Examiner Tips and Tricks

Don’t worry about remembering the values of R and k, they will both be given in the equation sheet in your exam.

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

I would just like to say a massive thank you for putting together such a brilliant, easy to use website.I really think using this site helped me secure my top gradesin science and maths. You really did save my exams! Thank you.

Beth
IGCSE Student

This website is soooo useful and I can’t ever thank you enough for organising questions by topic like this. Furthermore, the name of the website could not have been more appropriate as it literally did SAVE MY EXAMS!

Fathima
A Level Student

Incredible! SO worth my money, the revision notes have everything I need to know and are so easy to understand. I actually enjoy revising! It makes me feel a lot more confident for my GCSEs in a few months.

Kate
GCSE Student

Absolutely brilliant, both my girls used it for A levels and GCSE. It's saves on paper copies, also beneficial exam questions ranked from easy to hard. It's removed a lot of stress from the exams.

Sameera
Parent

Just to say that your resources are the best I have seen and I have been teaching chemistry at different levels for about 40 years

Mark
Chemistry Teacher

Excellent