What does the Z factor describe in gas behavior?

• A. The ratio of the actual gas volume to the volume of an ideal gas at the same pressure and temperature
• B. The ratio of gas density to air density
• C. The ratio of gas temperature to ambient temperature
• D. The ratio of gas viscosity to air viscosity

Answer: (A)

Z describes how much a real gas’s volume deviates from an ideal gas at the same pressure and temperature. It’s a dimensionless factor defined by Z = (P V_m) / (R T) for molar volume V_m, so it also means V_m = Z R T / P. In the ideal gas law, PV = RT (per mole), so Z would be 1. Real gases have intermolecular attractions and finite molecular size, which cause their actual volume to differ from the ideal prediction. If Z is close to 1, the gas behaves nearly ideally. If Z is greater than 1, the gas occupies more volume than an ideal gas would at those conditions (repulsive forces dominate); if Z is less than 1, it occupies less volume (attractive forces dominate).