for a real gas at 25 ° C temperature and high pressure ( 99 bar)...

For real gas under high pressure we can neglect pressure corrections due to intermolecular forces. But since volume will be low, we can not neglect volume corrections due to molecular size.

$Z = 1 + \dfrac{Pb}{RT} = 2 ⇒ b = \dfrac{RT}{P}$

$= \dfrac{0 . 083 \times 298}{99}$

$= 0 . 25 \times {10}^{- 2}L{mol}^{- 1}$