Three moles helium and one mole neon are mixed at constant temperature and pressure, 25°C and 1 atm. Find mixing entropy in J/K to 2 decimals. R = 8.314 J/(mol K).
For ideal gases, $\Delta S_{mix}=-R\sum n_i\ln x_i$. Mole fractions are 3/4 and 1/4, so $\Delta S=-8.314[3\ln(0.75)+\ln(0.25)]=18.70$ J/K. The mixing entropy is positive even with unchanged temperature and pressure.
More Equilibrium: Laws of thermodynamics. Standard states. Thermochemistry. Thermodynamic functions and their relationships: Gibbs-Helmholtz and Maxwell relations, Gibbs-Duhem equation, Van’t Hoff equation. Criteria of spontaneity and equilibrium. Absolute entropy. Partial molar quantities. Thermodynamics of mixing. Chemical potential. Fugacity, Activity and Activity coefficient. Chemical equilibria. Dependence of equilibrium constant on temperature and pressure. questions
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