In the Cu-O-S system at 25⁰C and 1 atm. total pressure, calculate the partial pressures of O₂ and S₂ at coexistence of the solid phases cuprite (Cu₂O), tenorite (CuO), and chalcocite (Cu₂S). The phase predominance diagram for this system is shown in Fig. 9.8 in Geochemical Thermodynamics, 2nd ed., D. K. Nordstrom and J. L. Munoz, Blackwell Scientific Publications, Oxford, 1994.

This example problem, included in all versions of EQS4WIN, considers the system consisting of the gaseous species S₂2, and the solids Cu₂, CuO, Cu₂S, CuS, and Cu. The initial problem is set up to calculate the partial pressures of the gaseous species at the invariant point where the 3 phases coexist. This calculation illustrates the Equilibrium Constraints feature of EQS4WIN. The problem parameters may also be altered to calculate other system properties.