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The ideal gas equation



Amount of Substance

AQA Content

Use the equation in calculations.
Students will not be expected to recall the value of the gas constant, R.

Specification Notes

The ideal gas equation pV = nRT with the variables in SI units.


Learn to use and manipulate the following equation:

There's a couple of things to bear in mind when using this equation:

Pressure is in pascals not kilopascals.
Volume is in cubic metres, 1 cubic metre = 1 000 000 cubic centimetres
Temperature is in kelvin, 0°C = 273 K
n = number of moles
R is the Gas Constant

The ideal gas equation relates to a hypothetical ideal gas. To use the equation for real gases, we have to make some assumptions, including:

Gases are considered to be made up of sold spheres that move in straight lines. Their direction is random and they are constantly moving.
After collisions, kinetic energy is the same as before the collision.
Temperature is directly proportional to the kinetic energy of the particles.

Molar gas volume
At 25°C (298 K), atmospheric pressure (p = 101 325 Pa) and using a gas constant, R = 8.31 J K−1 mol−1, the volume of 1 mole (n = 1) of a gas is calculated as:

pV = nRT
V = nRT/p = (1 × 8.31 × 298)/101325 = 0.0244m3
V = 24.4 dm3

24 dm3 is used as the molar gas volume for 1 mole of any gas at 25°C and atmospheric pressure.


Exam Question


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