When does the graph of $p \propto \frac{1}{V}$ not result in a straight line?

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 $'b'$?

$A$ certain quantity of real gas occupies a volume of $0.15 \ dm^3$ at $100 \ atm$ and $500 \ K$ when its compressibility factor is $1.07$. Its volume at $300 \ atm$ and $300 \ K$ (when its compressibility factor is $1.4$) is $........ \times 10^{-4} \ dm^3$ (Nearest integer).

At high pressure,the form of Van der Waals equation becomes

For a real gas at $25^{\circ} C$ temperature and high pressure $(99 \ bar)$,the value of the compressibility factor is $2$. The value of the Van der Waals constant '$b$' is $\times 10^{-2} \ L \ mol^{-1}$. (Nearest integer) (Given $R = 0.083 \ L \ bar \ K^{-1} \ mol^{-1}$)

The Van der Waals constant '$a$' for the gases $O_2$,$N_2$,$NH_3$ and $CH_4$ are $1.3$,$1.390$,$4.170$ and $2.253 \ L^2 \ atm \ mol^{-2}$ respectively. The gas which can be most easily liquefied is