What is the unit of the Van der Waals constant $a$?

The compressibility factor of a real gas at high pressure is

At low pressure,the Van der Waals equation is reduced to:

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

If the inversion temperature of a gas is $-80\,^{\circ}C$,then it will produce cooling under the Joule-Thomson effect at ............... $K$.

At $300 \ K$,the compressibility factor of $1 \ mole$ of a gas is $1.1$. Its pressure is $2.706 \ atm$. What is its volume in $L$? (Given $R=0.082 \ L \ atm \ mol^{-1} \ K^{-1}$).