What is the equilibrium constant expression for the reaction?

${\mathrm{P}}_4\left(s\right)+5{\mathrm{O}}_2\left(g\right)\rightleftharpoons {\mathrm{P}}_4{\mathrm{O}}_{10}\left(s\right)?$

$P_4\left(\mathrm{s}\right)+5{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{k_c}{\rightleftharpoons }{P}_4{O}_{10}\left(\mathrm{g}\right)$

As we know, during. writing equilibrium expression, concentration of pure solid and liquid are taken as unity.

So, 

$\left[P_4\right]=1, \left[P_4{O}_{10}\right]=1$

$\Rightarrow K_c=\frac{\left[P_4{O}_{10}\right]}{\left[P_4\right]{\left[O_2\right]}^5}$

$=\frac{1}{1{\left[O_2\right]}^5}$

$K_c=\frac{1}{{\left[O_2\right]}^5}$