For which of the following is the van der Waals equation valid?

At high temperature and low pressure,the Vander Waal's equation is reduced to

For real gases,the relation between $P$,$V$ and $T$ is given by the van der Waals equation,$(P + \frac{an^2}{V^2})(V - nb) = nRT$. For the following gases $CH_4, CO_2, O_2, H_2$,which gas will have $(i)$ highest value of $'a'$ $(ii)$ lowest value of $'b'$?

When is deviation more in the behaviour of a gas from the ideal gas equation $PV = nRT$?

The cooling in a refrigerator is due to

What are the values of $Z$ for $CO_2, CH_4$,and $O_2$ at $200 \ bar$ and $900 \ bar$ pressure?