AQA A Level Chemistry

Revision Notes

5.3.2 Partial Pressure

Partial Pressure

  • The partial pressure of a gas is the pressure it exerts in a mixture of gases if it occupied the container on its own
  • Partial pressure is given the symbol p, so for a gas X, it is written as pX
  • The total pressure is the sum of the partial pressures (this is known as Daltons’ Law)

Partial Pressures, downloadable AS & A Level Chemistry revision notes

  • The mathematical relationships are as follows

Partial pressure and mole fraction formulae, downloadable AS & A Level Chemistry revision notes

Partial pressure and mole fraction expressions

Worked Example

Working out mole fractions

A sample of  0.25 mole of nitrogen and 0.75 mole of hydrogen were reacted together to form ammonia. The equilibrium amount of nitrogen was 0.16 mole.

N2 (g) + 3H2 (g)   2NH3 (g)

Calculate the mole fractions of nitrogen, hydrogen and ammonia.


Write out the equation and record the initial, the change and the equilibrium amounts:

WE Mole Fractions Answer, downloadable AS & A Level Chemistry revision notes

Exam Tip

You can check you have the mole fractions correct by adding them up and making sure they come to 1:

0.195 + 0.585 + 0.220 = 1

Kp Calculations

  • Kp calculations are a step-by-step process in which you need to find
    • the mole fractions of the gases present
    • their partial pressures
    • the Kp expression
    • the value of Kp
  • The following worked example shows how this is achieved

Worked Example

Finding Kp

Hydrogen and bromine were mixed in a flask in a 1: 1 ratio and allowed to reach equilibrium at 450 K. When equilibrium had been achieved the total pressure in the flask was 140 kPa and the mole fraction of bromine was 0.35.

The equation for the reaction is

H2 (g) + Br2 (g)  ⇌ 2HBr (g)

Determine the partial pressures for each gas at equilibrium and the value of Kp

WE1 Answer, downloadable AS & A Level Chemistry revision notes

  • Another style of Kp calculation involves being given the value of Kp and working backwards to deduce the partial pressure of one of the gases

Worked Example

Finding Partial Pressure

Kp for the dissociation equilibrium reaction of nitrogen monoxide is 0.0021.

2NO (g)  ⇌ N2 (g) + O2 (g)

If pure NO is introduced into a reaction flask at an initial pressure of 100 kPa, what is the equilibrium partial pressure of nitrogen?


WE2 Answer 1, downloadable AS & A Level Chemistry revision notes

WE2 Answer 2, downloadable AS & A Level Chemistry revision notes


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