Question

The equilibrium p4 (s) + 6cl2 (g) â 4pcl3(g) is attained by mixing equal moles of p4 and cl2 in an evacuated vessel. Then at equilibrium

Answer 1

When the equilibrium P₄ (s) + 6Cl₂ (g) ⇒ 4PCl₃(g) is attained by mixing equal moles of P₄ and Cl₂ in an evacuated vessel you can use the information to calculate the equilibrium constant by using the equilibrium concentrations of phosphorus, chlorine and phosphorus trichloride.

But being that your question seems to be incomplete all you need to do concerning this question is that determine the initial concentration, change in concentration and the equilibrium concentrations.  

After which you can use the equilibrium concentration to calculate the equilibrium constant.