Step 1 - Understanding the equilibrium constant
For a chemical reaction:
[tex]aA+bB\rightleftarrows cC+dD[/tex]The equilibrium constant would be:
[tex]K=\frac{\lbrack C\rbrack^c\lbrack D\rbrack^d}{\lbrack A\rbrack^a\lbrack B\rbrack^b}[/tex]We need to be careful because pure liquids and solids do not enter the equilibrium constant.
Step 2 - Finding the equilibrium constant for the given reaction
For this reaction:
[tex]4\text{HCl}_{(g)}+O_{2(g)}\rightleftarrows2H_2O_{(g)}+2Cl_{2(g)}[/tex]We can see that all substances are in the gaseous state. Therefore, all substances will enter the equilibrium constant. The equilibrium constant in this case will be:
[tex]K=\frac{\lbrack C\rbrack^c\lbrack D\rbrack^d}{\lbrack A\rbrack^a\lbrack B\rbrack^b}=\frac{\lbrack H_2O\rbrack^2\lbrack Cl_2\rbrack^2}{\lbrack O_2\rbrack^{}\lbrack HCl\rbrack^4}[/tex]The correct answer is thus alternative e).
