Since the slow step is the rate determining step hence if we consider option (A) we find Rate =k[Cl2][H2S] Now if we consider option (B) we find Rate=k[Cl2][HS−]....(i) For equation, H2S⇌H++HS− K=
[H+][HS−]
H2S
or [HS−]=
K[H2S]
H+
Substituting this value in equation (i) we find Rate
=k[Cl2]K
[H2S]
H+
=k′
[Cl2][H2S]
[H+]
Thus slow step should involve 1 molecule of Cl2 and 1 molecule of H2S. hence only , mechanism (A) is consistent with the given rate equation