Discussion Forum

For  the elementary reaction M→ N,  the rate of disappearance of M increases by  a factor of 8 upon doubling   the concentration of M. The order of the reaction with respect to M is

Answer:

Explanation:

For the elementary reaction, M→N

 Rate law can be written as

          Rate   $\propto [M]^{n}$

   Rate = k[M]ⁿ       ...........(i)

 When we double the concentration of  [M]  , rate becomes  8 times , hence  new rate law can be written as

       $8\times Rate =k[2M]^{n}$        ...........(ii)

   $\frac{Rate}{8\times Rate }=\frac{k[M]^{n}}{k[2M]^{n}}$

$\Rightarrow$     $  \frac{1}{8}=\frac{1}{[2]^{n}}\Rightarrow  [2]^{n}=8=[2]^{3}$

           n=3

 Hence, (b) is the correct choice.