What is the equation for the Z factor?

• A. Z = VIDEAL / VACTUAL
• B. Z = VACTUAL / VIDEAL
• C. Z = P / T
• D. Z = Vgas / Vliquid

Answer: (B)

Z factor shows how a real gas’s volume compares to what an ideal gas would occupy at the same temperature and pressure. The direct way to express this is Z = V_actual / V_ideal, because it compares the actual molar volume to the ideal molar volume. Since V_ideal = RT/P for one mole, this ratio is also Z = (P × V_actual) / (R × T). That ties the concept to the familiar PV/RT form, showing why this ratio is the correct definition.

Z = V_ideal / V_actual would invert the meaning, giving values that don’t reflect deviation in the correct way. Z = P / T ignores volume altogether and isn’t a measure of non-ideal behavior. Z = V_gas / V_liquid compares different phases, not the deviation of a gas from ideal behavior.