A single equation of state (EOS), such as the Peng-Robinson EOS, can accurately describe both the liquid and vapor phase. The Peng-Robinson EOS is part of a family of equations of state called cubic because the compressibility factor, , is a solution of a cubic equation. This Demonstration performs flash calculations of a mixture of hydrocarbons (2.5% ethane, 35% propane, 60% n-butane, and 2.5% n-pentane). It uses the Rachford-Rice equation. The phi-phi method, based on the Peng-Robinson EOS, is applied to compute the equilibrium constants. Once you set the temperature, the program computes the corresponding vapor fraction and the liquid and vapor compositions at .