Four variables define the physical behaviour of a sample of gas: pressure (P), volume (V), temperature (T), amount / no. of moles of gas (n);

There are 4 Rules about Pressure:

  • The Ideal Gas Law


* R is a constant - there are two variants depending on the required units
* The whole point of units is to cancel out leaving the correct unit for the value we are finding.

  • Subset Laws
    • The other three laws are subsets of the Ideal Gas Law, for when 2 of the 4 variables are fixed.
    • Boyle's Law
      • Volume inversely proportional to Pressure
    • Charles' Law
      • Volume proportional to Temperature
    • Avogadro's Law
      • Volume proportional to Moles
      • i.e. equal volumes contain equal numbers of moles



Units are super important. Units must be consistent and of the right type:

  • Temperature: Kelvin (273K = 0 Celsius)
  • Pressure: Atms or Pascals. Atms corresponds to L, Pascals to Metres Cubed
  • Volume: L or M3
  • Moles: A number amount
  • R: The correct value/unit pair for the other units.

Partial Pressure

In a uniform mix of gases at equilibrium (i.e. no gases are reacting), each individual gas has a partial pressure - the pressure it would exert if it were the only gas in the same conditions (V, T, n, etc).

Total pressure is the sum of all the partial pressures, and Partial pressure of a component in a gas mixture is its mole fraction multiplied by the total pressure. (Fiven that all gases act the same under the same conditions, so Pressure is dependent on moles)