Final answer:
The equilibrium-constant expression for the evaporation of water is the partial pressure of water vapor.
Step-by-step explanation:
The question is asking to write the equilibrium-constant expression for the evaporation of liquid water to water vapor in terms of partial pressures.
The equilibrium-constant expression for a reaction is given by the ratio of the product of the partial pressures of the products raised to their stoichiometric coefficients divided by the product of the partial pressures of the reactants raised to their stoichiometric coefficients.
Since the evaporation of water can be written as H2O(l) ↔ H2O(g), the equilibrium constant (Kp) expression for this physical process is simply the partial pressure of water vapor, PH2O(g).