CHEMISTRY COMMUNITY

This question gives a chemical equation and asks you to write the expression for K, and the solutions manual uses P subscript molecular formula rather than just the molecular formula and concentrations. Are these two forms interchangeable when the question doesn't specifically ask for concentrations or to use partial pressures?

If the question just asks for K, you can use either, but it is probably better to use what you are given. In this case, I am sure you were given partial pressure which is why you got a Kp value. However, if the question asked for Kc, you would need to convert your partial pressures to concentration through pv=nrt, given you know r and t values.

Also, it could depend on what phase your reactants and products are in if you are not given actual values.

The solution is just writing out the chemical equilibrium constant in the form of Kp rather than Kc.

They are interchangeable for gases only.

When writing it with P subscripts, this indicates that it is Kp, with the equilibrium partial pressures of products on top divided by the equilibrium partial pressures of reactants on the bottom, both raised to their stoichiometric coefficients.

Meanwhile, Kc utilizes equilibrium molar concentrations.

So if the chemical equilibrium equation consists only of gases and K isn't specified as Kc or Kp, you'd most likely use P subscripts?

You can use either one.