The reaction of formation of phosgene from $CO$ and $Cl_2$ is $CO + Cl_2 \to COCl_2.$ The proposed mechanism is
$(i)$ $Cl_2 \,\underset{k_2}{\overset{k_1}{\longleftrightarrow}}\, 2Cl$
$(ii)$ $Cl + CO \,\underset{k_4}{\overset{k_3}{\longleftrightarrow}}\, COCl$
$(iii)$ $COCl + Cl_2 \xrightarrow{k_5} COCl_2 + Cl$ (slow)
Find the correct expression of rate law.

• A
  $r = k_5 \times \frac{k_3}{k_4} \times \left( \frac{k_1}{k_2} \right)^{1/2} [CO][Cl_2]^{3/2}$
• B
  $r = k_5 \times \frac{k_3}{k_4} \times \left( \frac{k_1}{k_2} \right)^{1/2} [CO][Cl_2]^{1/2}$
• C
  $r = k_5 \times \left( \frac{k_3}{k_4} \right)^{1/2} \times \frac{k_1}{k_2} [CO][Cl_2]^{3/2}$
• D
  None of these