# For which gases does the ratio of PV to RT equal a constant?

By definition, $\frac{P V}{R T} = Z$ is the compressibility factor. When $Z > 1$, the gas's repulsive forces dominate (it takes up more molar volume than its ideal counterpart), and when $Z < 1$, the gas's attractive forces dominate (it takes up less volume than its ideal counterpart).