Either density or this quantity is used as the variable of integration when calculating residual properties. A truncated virial equation sets the compressibility factor equal to one plus b times this quantity. Cubic equations of state are written as explicit functions of this quantity. Enthalpy equals heat only if this quantity does not change for 
Either density or this quantity is used as the variable of integration when calculating residual properties. A truncated virial equation sets the compressibility factor equal to one plus b times this quantity. Cubic equations of state are written as explicit functions of this quantity. Enthalpy equals heat only if this quantity does not change for an open system. The fugacity coefficient is defined as fugacity over this quantity. (*) This quantity equals the liquid-phase concentration times Henry's constant for dilute solutions. Work is the area under a graph plotting this quantity versus volume. This quantity times volume has units of energy, and it multiplies volume in the ideal gas law. For 10 points, name this quantity measured in bars, torr, or atmospheres.