What is the equilibrium expression for the reaction P4(s) + 5O2(g) ⇌ P4O10(s) ?

Kc = [O2]⁵
Kc = [P4O10]/5[P4][O2]
Kc = [P4O10]/[P4][O2]⁵
Kc = 1/[O2]⁵

Correct answer: Kc = 1/[O2]⁵

Solution

The equilibrium expression for a reaction only includes the concentrations of gaseous and aqueous species, while solids and liquids are omitted. In this case, since P4 and P4O10 are solids, the expression focuses on the gaseous reactant O2, leading to the correct form of Kc as the inverse of its concentration raised to the power of its stoichiometric coefficient.

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