= Solution
For the one-mole convention, \b[$b$ is the excluded <molar volume>]: finite molecular size reduces the volume available for translational motion from $V$ to $V-b$. It is an effective <excluded volume>, not simply the sum of the bare molecular volumes. \b[$a/V^2$ corrects for attractive <intermolecular forces>], which reduce the <pressure> exerted on the wall. The attraction per molecule scales with <number density>, and the number of affected molecules also scales with <density>, producing the density-squared correction. The <Van der Waals equation> consequently raises the measured <pressure> to the corresponding repulsive-gas value before multiplying by the available volume. Both corrections vanish relative to the ideal-gas terms at sufficiently low <density>.
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