The unit of the rate constant for an $n^{th}$ order reaction is .....

Order of a reaction is decided by

For the reaction $A + B \rightarrow$ products,it is observed that
$(i)$ on doubling the initial concentration of $A$ only,the rate of reaction is also doubled and
$(ii)$ on doubling the initial concentration of both $A$ and $B$,there is a change by a factor of $8$ in the rate of the reaction.
The rate of this reaction is given by

Which of the following statements is incorrect?

The rate constant for the reaction,$2N_2O_5 \to 4NO_2 + O_2$ is $3 \times 10^{-5} \, s^{-1}$. If the rate is $2.40 \times 10^{-5} \, mol \, L^{-1} \, s^{-1}$,then the concentration of $N_2O_5$ (in $mol \, L^{-1}$) is

Why can’t molecularity of any reaction be equal to zero?