What does the compressibility factor Z quantify in gas behavior?

• A. The ratio of gas constant to temperature
• B. The ratio of real molar volume to ideal molar volume
• C. The ratio of real to ideal gas pressure
• D. The ratio of liquid to vapor pressure

Answer: (B)

Compressibility factor Z quantifies how much a real gas deviates from ideal gas behavior by comparing the actual molar volume to the ideal molar volume at the same pressure and temperature. It is defined as Z = P V_m / (R T), which is equivalently Z = V_m(real) / V_m(ideal) since V_m(ideal) = R T / P for an ideal gas. If Z = 1, the gas behaves ideally. If Z < 1, attractive intermolecular forces make the gas occupy less volume than an ideal gas would at the same conditions. If Z > 1, repulsive interactions or finite molecular size cause the gas to occupy more volume. Z varies with P and T and is central to correcting ideal-gas predictions with real-gas behavior.