Consider the reaction :
$$C{l_2}\left( {aq} \right) + {H_2}S\left( {aq} \right) \to $$     $$S\left( s \right) + 2{H^ + }\left( {aq} \right) + 2C{l^ - }\left( {aq} \right)$$
The rate equation for this reaction is
rate $$ = k\left[ {C{l_2}} \right]\left[ {{H_2}S} \right]$$
Which of these mechanisms is/are consistent with this rate equation?
$${\text{A}}{\text{.}}\,\,C{l_2} + {H_2}S \to {H^ + } + C{l^ - } + C{l^ + }$$       $$ + H{S^ - }\left( {{\text{slow}}} \right)$$
$$C{l^ + } + H{S^ - } \to {H^ + } + C{l^ - } + S\left( {{\text{fast}}} \right)$$
$${\text{B}}{\text{.}}\,\,{H_2}S \rightleftharpoons {H^ + } + H{S^ - }$$     $$\left( {{\text{fast equilibrium}}} \right)$$
$$C{l_2} + H{S^ - } \to 2C{l^ - } + {H^ + } + S\left( {{\text{slow}}} \right)$$

Solution :
Since the slow step is the rate determining step hence if we consider option (A) we find
$${\text{Rate}} = k\left[ {C{l_2}} \right]\left[ {{H_2}S} \right]$$
Now if we consider option (B) we find
$$\eqalign{ & {\text{Rate}} = k\left[ {C{l_2}} \right]\left[ {H{S^ - }} \right]\,\,\,\,\,...\left( {\text{i}} \right) \cr & {\text{From equation (i)}} \cr & k = \frac{{\left[ {{H^ + }} \right]\left[ {H{S^ - }} \right]}}{{{H_2}S}}\,\,{\text{or}}\,\left[ {H{S^ - }} \right] = \frac{{k\left[ {{H_2}S} \right]}}{{{H^ + }}} \cr} $$
Substituting this value in equation (i) we find
$${\text{Rate}} = k\left[ {C{l_2}} \right]K\frac{{\left[ {{H_2}S} \right]}}{{{H^ + }}} = k'\frac{{\left[ {C{l_2}} \right]\left[ {{H_2}S} \right]}}{{\left[ {{H^ + }} \right]}}$$
hence only , mechanism (A) is consistent with the given rate equation.