Which of the following can be most easily liquefied? (Value of $a$ for $NH_3 = 4.17, CO_2 = 3.59, SO_2 = 6.71, Cl_2 = 6.49$)

The correction factor '$a$' in the van der Waals equation for real gases corresponds to:

One mole of a monoatomic real gas satisfies the equation $p(V-b)=RT$ where $b$ is a constant. The relationship of interatomic potential $V(r)$ and interatomic distance $r$ for the gas is given by

$A$ real gas would be most likely to obey the ideal gas laws at

Between $NH_3$ and $N_2$,which one has a higher value of $(i) \ a$ and which one has a higher value of $(ii) \ b$? (Where $a$ and $b$ are van der Waals constants.)

Assertion : Compressibility factor $(Z)$ for non-ideal gases can be greater than $1$.
Reason : Non-ideal gases always exert higher pressure than expected.