In the van der Waals equation,which of the following will have the highest value of '$a$'?

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.)

Given van der Waals constant $a$ for $NH_{3}$,$H_{2}$,$O_{2}$ and $CO_{2}$ are respectively $4.17$,$0.244$,$1.36$ and $3.59$,which one of the following gases is most easily liquefied?

The compressibility factor $(Z)$ of a gas is less than unity at $STP$,then:

Pressure exerted by $1 \, mole$ of methane in a $0.25 \, L$ container at $300 \, K$ using van der Waals equation is ............... $atm$ (given $a = 2.253 \, atm \, L^2 \, mol^{-2}, b = 0.0428 \, L \, mol^{-1}$):

Under which conditions do real gases deviate most from ideal gas behavior?