Discussion
A mixture of carbon-dioxide (CO2), nitrogen (N2) and oxygen (O2) are introduced in a rigid and impermeable tank containing only liquid water (H2O). Assume that the components are non-reacting and un-dissociated. The system is kept isolated till an equilibrium is achieved, where only two phases are present. The degrees-of-freedom of the equilibrium mixture, as obtained from the phase rule, is
Answer: D
The degrees of freedom are 4.
Gibbs' phase rule for a non-reacting system is F = C - P + 2. There are four independent components (CO2, N2, O2 and H2O) and two phases (a gas phase and a liquid phase), so:
\[ F = 4 - 2 + 2 = 4 \]
Each gas dissolves to some extent in the water and water vapour enters the gas, so all four components are present in both phases, but that does not change C. Counting only the three gases as components gives 3; treating the system as a single phase gives 5, not offered.
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