For a reaction, $\mathrm{A} \longrightarrow \mathrm{B}$, rate equation is $\mathrm{r}=\mathrm{k}[\mathrm{A}]^{\circ}$. If initial concentration of reactant is 'a'mol dm ${ }^{-3}$ find half life time of reaction.

Which of the following compounds is formed as major product in the following reaction.

2-Methylbut-2-ene $\xrightarrow[\text { Peroxide }]{\mathrm{HBr}}$ Product

Which from following cations forms least stable complex with same ligand?

Which of the following species is not tetrahedral?