Heterogeneous equilibrium

[Artin Allahverdian 2H]

Lets say we have a chemical reaction at equilibrium where one of products/reactants is a gas and another one of the products/reactants is aqueous. When calculating the equilibrium constant for a specific case like this, would we just use the concentration of the aqueous reactant/product and the partial pressure of the gaseous reactant/product together in the same calculation for the equilibrium constant (Kc)?

[Elisa Bass 4L]

You would have to convert one of the given values to match the units of the other. Because an aqueous solution does not have a partial pressure, you should convert the partial pressure of the gas to a concentration.

[gwynlu1L]

You will either be finding Kc or Kp in a given question. Usually, the question will list concentrations if it wants you to find Kc, but you do not want to mix up using concentrations and partial pressures. When you think about it, the two values are in different units, so putting them together in the same calculation for the equilibrium constant doesn't make sense.

[Laurence Tacderas 1K]

You would have to convert either the partial pressure of the gas to equilibrium molar concentration to determine Kc or convert the equilibrium molar concentration of the aqueous solution to determine Kp. However, molarity and partial pressures cannot be used in the same equilibrium constant expression.