Identify least stable species from following.

Which instrument is used for determination of particle size?

Identify ' $A$ ' in the following reaction.

$$ \begin{aligned} &\mathrm{A}+\text { chloromethane } \xrightarrow[\mathrm{AlCl}_3]{\text { Anhydrous }} \text { 2-chlorotoluene }+4 \text {-chlorotoluene } \end{aligned} $$

What is the average rate of reaction when change in concentration of product is 0.05 M in 20 seconds?