Question

Explain how H2 and O2 differ from an ideal gas at higher pressures.

Answer 1

We can calculate the molar volume of the ideal gas at the same temperature and pressure.

Z=PV​/nRT

This ratio is called the compression factor, Z. For a gas with ideal behavior, Vm of the H2 and O2 gases are the same as Vm of an ideal gas equals, 1.

For the real gases like H2 and O2 are sometimes less than 1 at very low pressures, which tells us that the molar volume is less than that of an ideal gas. As you increase the pressure past a certain point that depends on the gas, Z gets increasingly larger than 1. That is, at high pressures the Vm, of the H2 and O2 gases are larger than Vm of the ideal gas, and Vm of the real gas increases with pressure.

At high pressures, the gas molecules get more crowded and the amount of empty space between the molecules is reduced. It helps to remember that the volume we use in the ideal gas equation is the empty volume that the gas molecules have to move around in. We usually assume that this is the same as the volume of the container when the gas molecules don’t take up much space.

For a given pressure, the real gas will end up taking up a greater volume than predicted by the ideal gas law since we also have to take into account the additional volume of the gas molecules themselves. This increases our molar volume relative to an ideal gas, which results in a value of Z that is greater than 1. The error in molar volume gets worse the more compressed the gas becomes, which is why the difference between Z for the real (H2 and O2) and ideal gas increases with pressure.