The Ideal Gas Law Explained

The ideal gas law ties together the four things you can measure about a gas — pressure, volume, amount and temperature — in one equation.

P is pressure, V is volume, n is moles, T is absolute temperature, and R is the gas constant. Rearrange it to solve for whichever quantity you are missing.

Choosing the right R

R has different numerical values depending on the units, so match it to your pressure and volume:

• R = 0.08206 L·atm·mol⁻¹·K⁻¹ with pressure in atm and volume in litres (the most common choice).
• R = 8.3145 J·mol⁻¹·K⁻¹ with SI units (pascals, cubic metres) — used in energy and kinetic-theory work.

The simplest habit: convert pressure to atm and volume to litres, then use 0.08206.

Temperature must be in kelvin

The law uses absolute temperature, so always convert: T(K) = T(°C) + 273.15. Forgetting this is the single most common mistake.

Worked example

How many moles of gas occupy 18.5 L at 11.2 atm and 28.2 °C?

When the gas changes conditions

If a fixed amount of gas moves between two states, the combined gas law P₁V₁ ÷ T₁ = P₂V₂ ÷ T₂ is quicker, because R and n cancel.

Solve any of P, V, n or T on the Ideal Gas Law Calculator — it converts your units automatically.