What is the compressibility factor $(Z)$ for the deviation of real gases from ideal behavior?

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

At $300 \ K$,one mole of a gas present in a $10 \ L$ flask exerted a pressure of $2.706 \ atm$. What is its compressibility factor $(Z)$? (Given $R=0.082 \ L \ atm \ mol^{-1} \ K^{-1}$).

For a real gas,the ratio $\frac{T_{C} \text{ (Critical temperature)}}{P_{C} \text{ (Critical pressure)}}$ is equal to:

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

For which of the following gases is the value of the van der Waals constant $'a'$ expected to be the highest?