Question

what is critical temperature? how is it related to van der waal constant 'a'

Answer 1

Critical Temperature (T_c):
It is the temperature above which a gas cannot be liquefied by applying pressure.
$$\begin{gathered} CriticaltemperatureisrelatedtovanderWaal\text{'}scons\tan taas \\ {T}_c=\frac{8a}{27Rb} \end{gathered}$$