[High school - Chemistry] Equilibrium Concepts

[Xception704]

So this is the problem:

Write expressions for K for the following reactions:

a. P4 (s) + 5O2 (g) <----> P4O10 (s)

The equation for an equilibrium expression is K = [C]^c[D]^d
.................................................. ..............[A]^a[b]^b

Using this formula, I thought the answer would have been [P4O10]
.................................................. ...........................[P4][O2]^5

The back of the book says 1/[O2]^5

Anyone willing to explain to me why that is? I thought about if maybe they'd cancel, but that'd leave O10/[O2]^5

=\

[MeaCulpa]

Solids and pure liquids don't show up in equilibrium expressions. It has something to do with the concept of activities and other thermodynamics stuff, but don't worry about that much. Just know that only concentrations of aqueous solutions and partial pressures (or concentrations) of gases are used in deriving expressions for K.

[Xception704]

Oh wow. That's what I get for not reading my notes. Thanks. =)