<u>Answer:</u> The expression of
is written below.
<u>Explanation:</u>
Equilibrium constant is defined as the ratio of concentration of products to the concentration of reactants each raised to the power their stoichiometric ratios. It is expressed as 
For a general chemical reaction:

The expression for
is written as:
![K_{eq}=\frac{[C]^c[D]^d}{[A]^a[B]^b}](https://tex.z-dn.net/?f=K_%7Beq%7D%3D%5Cfrac%7B%5BC%5D%5Ec%5BD%5D%5Ed%7D%7B%5BA%5D%5Ea%5BB%5D%5Eb%7D)
Concentration of solid and liquid substances in a chemical reaction is taken to be 1.
For the given chemical equation:

The expression of
for the above equation follows:
![K_{eq}=\frac{[Cl_2]\times [H_2]}{[HCl]^2}](https://tex.z-dn.net/?f=K_%7Beq%7D%3D%5Cfrac%7B%5BCl_2%5D%5Ctimes%20%5BH_2%5D%7D%7B%5BHCl%5D%5E2%7D)
Hence, the expression of
is written above.