What is the order of a chemical reaction that obeys the expression ${\mathrm{t}}_{1/2}=\frac{1}{\mathrm{k}.\mathrm{a}}?$

$t_{1/2}$ for ${\mathrm{n}}^{\mathrm{th}}$ order reaction $=\frac{2^{\mathrm{n}-1}-1}{(\mathrm{n}-1)\mathrm{k}{\left(\mathrm{a}\right)}^{\mathrm{n}-1}}$

On substituting n = 2, we obtain 

${\mathrm{t}}_{1/2}=\frac{2-1}{1.\mathrm{k}.\mathrm{a}}=\frac{1}{\mathrm{ka}}$

Therefore, the order of the reaction is 2.