BITSAT 2016 Paper

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
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