Equilibrium Constant for Homogenous Systems - Gas Partial Pressures (A-Level Chemistry)

Gas Partial Pressures

Partial Pressure

Partial pressure is the contribution to pressure a single gas makes to the total pressure in a mixture of gases.

A mole fraction is the proportion of total gas moles in a gas mixture that corresponds to a particular gas.

Gas Partial Pressures
Gas Partial Pressures

We can work out the partial pressure of a gas, A, in a mixture of gases using the chemical equation of the reversible reaction and the following formulae:

Worked example: The following reaction reaches equilibrium,

The mole fraction of NH3 at equilibrium is 0.60 and the total pressure is 160 kPa. Nitrogen and hydrogen are mixed in a 1:3 ratio.

Calculate the partial pressure of each gas in the equilibrium mixture (3 marks)


Partial pressure of ammonia = 0.60 x 160 = 96 kPa (1)

1. Work out the mole fractions hydrogen and nitrogen

Mole fraction of hydrogen + nitrogen = 1- 0.60 = 0.40

The ratio of nitrogen : hydrogen = 1:3

Therefore mole fraction of nitrogen = 0.1

Mole fraction of hydrogen = 0.3

2. Work out the partial pressures of nitrogen and hydrogen

Partial pressure of nitrogen = 0.1 x 160 = 16 kPa (1)

Partial pressure of hydrogen = 0.3 x 160 = 48 kPa (1)1

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