Pressure of a gas

In gases, the particles are far apart from each other. They're constantly moving at speed, and in a random manner. The particles continually collide with each other and the walls of the container, bouncing off in all directions so they fill the container. It's the molecules in collision with the walls of the container that we measure as the pressure of the gas.

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Solids, liquids and gas animations are available in this downloadable resource, Particles in motion: Change of State - solid, liquid and gas, provided by the National STEM Centre and developed by the Royal Society of Chemistry and Gatsby Foundation as part of the Science Enhancement Programme.

Partial pressures

In a mixture of gases the molecules of each gas collide with the walls of the container and exert a pressure. We call the pressure of a gas in a mixture of gases its partial pressure. The sum of all the partial presures of gases in a mixture add up to the total pressure of the gaseous mixture.

Download the Phet animation, gas properties. 

Set the parameters to constant volume. Set the number of heavy particles to 30. What pressure is produced?

About 0.5 atm.

Increase the number of heavy particles to 60. What pressure is produced now?

About 1.0 atm.

Does the pressure of a gas depend upon the number of particles present?

Yes. The pressure exerted by a gas depends on the number of molecules in a given volume. The greater the number of molecules, the greater the pressure.

Now add 60 light molecules to the 60 heavy particles. What is the pressure now?

About 2.0 atm.

Does the pressure depend on the nature of the molecules in the gas?

No. The pressure depends upon the number of molecules, not on what they are.

What do you know about the number of molecules in 1 mole of any gas?

It is the same, 6.02 x 10²³ molecules.

So instead of talking about the numbers of molecules in a gas mixture we can talk about the number of moles of each gas.

If we have a mixture of 1 mole of two different gases at an overall pressure of 6 kPa, what will be the partial pressure of each gas?

3 kPa.

If we have a mixture of 1 mole of gas A and 2 moles of gas B with a total pressure of 6 kPa, why will the partial pressure of gas A be 2 kPa?

Gas A makes up one-third of the moles of gas or one-third of the molecules in the mixture so the partial pressure is one-third of the total pressure.

Mole fraction

The fraction of the total number of moles of gas for a particular gas is known as its mole fraction.

In a mixture of 1 mole of gas A, 2 moles of gas B and 3 moles of gas C, what are the mole fractions of the three gases?

Mole fraction of gas A = 1/6
Mole fraction of gas B = 2/6 = 1/3
Mole fraction of gas C = 3/6 = 1/2

If the total pressure of the mixture of gases A, B and C is 12 kPa, what are the partial pressures of each gas?

Partial pressure of gas A = 1/6 x 12 = 2 kPa
Partial pressure of gas B = 1/3 x 12 = 4 kPa
Partial pressure of gas C = 1/2 x 12 = 6 kPa

Write an equation to show the relationship between the partial pressure of a gas, the total pressure of the gas mixture and the mole fraction of the gas.

partial pressure = mole fraction x total pressure