Question

For real gases, how does a change in pressure affect the ratio of PV to nRT?

Answer 1

It depends on the gas. The ratio of `PV` to `nRT` is the compressibility factor `Z`:

`Z = (PV)/(nRT)`

• When `Z > 1`, the molar volume of the gas is larger than predicted by the ideal gas law, so the gas's repulsive intermolecular forces dominate.

• When `Z < 1`, the molar volume of the gas is smaller than predicted by the ideal gas law, so the gas's attractive intermolecular forces dominate.

• When `Z = 1`, the gas is ideal.

In principle, higher pressures (and lower temperatures) should make the gas behave more like a real gas (interacting, "sticky" particles).

But higher pressures alone don't give rise to a clear relationship with `Z`.

You can see that at higher temperatures, the curve for `"N"_2` converges upon `Z = 1` across a large range of pressures, demonstrating that very high temperatures give rise to an ideal gas. Having low pressures as well makes it easier to accomplish that feat.