A strong 2.00 L steel cylinder holds 4.00 mol of carbon dioxide at 300 K…

Step 1. The ideal pressure. First $nRT = 4.00\ \text{mol} \times 8.31\ \text{J/(mol K)} \times 300\ \text{K} = 9972\ \text{J}$. Then $p = \dfrac{nRT}{V} = \dfrac{9972\ \text{J}}{2.00 \times 10^{-3}\ \text{m}^3} = 4.99 \times 10^6$ Pa, that is $4.99$ MPa.

Step 2. The ratio. $\dfrac{pV}{nRT} = \dfrac{4.00\ \text{MPa}}{4.99\ \text{MPa}} = 0.80$.

Step 3. Which effect? The real pressure is lower than the ideal one. The molecules pull on each other, so they hit the walls less hard. The pull is winning. Carbon dioxide molecules pull on each other strongly, which is why carbon dioxide turns into a liquid easily when it is squeezed.