Which statement best defines the Z factor for a gas?

• A. Z equals the ratio of gas mass to gas volume
• B. Z equals the ratio of ideal gas volume to actual gas volume
• C. Z equals the ratio of actual gas volume to ideal gas volume at the same P and T
• D. Z equals the sum of volumes of real gas and ideal gas

Answer: (C)

Z shows how real gas behavior differs from an ideal gas by comparing volumes at the same pressure and temperature. It is defined as Z = PV/RT for a given amount of gas, which is also the ratio of the actual gas volume to the ideal gas volume at the same P and T. This framing lets you see that when Z equals 1, the gas behaves ideally; deviations indicate non-ideal effects from molecular interactions or finite volume. So stating that Z is the ratio of the actual gas volume to the ideal gas volume at the same P and T captures this definition. The other ideas don’t fit: using mass over volume describes density, inverting the ratio would give 1/Z, and summing volumes has no basis in the definition of Z.