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

To find the partial pressure of a gas, you need two pieces of information
- The total pressure in the container
- The mole fraction K_p
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 fractionsA 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.

Answer

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

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

Worked example

Finding K_pHydrogen 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 K_p

WE1 Answer, downloadable AS & A Level Chemistry revision notes