Compare the van der Waals, SRK, and Peng-Robinson equations of state.

Van der Waals (1873): P = RT/(V-b) - a/V^2; first to include molecular volume (b) and attraction (a). Simplest but inaccurate for liquid densities. SRK (Soave-Redlich-Kwong, 1972): improves attraction term with temperature-dependent alpha function based on acentric factor. Better vapor pressures but poor liquid densities. Peng-Robinson (1976): modified SRK with different critical compressibility, better liquid density prediction (Zc=0.307). PR widely used in petroleum/gas industry. Both SRK and PR are cubic, solving for three roots (vapor, liquid, or single phase).