How do I solve this problem on the reaction 4HCl+O2 <=>2Cl2+2H2O

Question

4HCl+O2 <=>2Cl2+2H2O (all gases)

 

Equal numbers of moles of HCl and O2 in a closed system are allowed to reach equilibrium as represented by the equation above. Which of the following must be true at equilibrium?

 

I. [HCl] must be less than [Cl2]

II. [O2] must be greater than [HCl]

III. [Cl2] must equal [H2O]

 

A. I only

 

B. II only

 

C. I and III only

 

D. II and III only 

 

Answer 1

If equal number of moles of HCl and O2 are used, then HCl will be limiting since it takes 4 moles of HCl per mole O2. This means @ equilibrium, [HCl] will be < [O2].  Also, since mole ratio of HCl : Cl2 and to H2O is 2:1, @ equilib. [Cl2] = [H2O] < [HCl].  So, looks like (D) II and III only would satisfy these conditions.