The equation of state for real gases is called van der Waals equation. Van der Waals equation for real gases is the corrected form of ideal gas equation which includes the effects of intermolecular forces of attraction and space occupied by gas molecules. We do not go into deriving van der Waals equation now but we can express it as

\[\left( {p + a\frac{{{n^2}}}{{{V^2}}}} \right)(V - nb) = nRT \tag{1} \label{1}\]

where \(a\) and \(b\) are constants. The constant \(a\) accounts for the fact that there is intermolecular force between molecules and the constant \(b\) accounts for the fact that molecules occupy some space of the container. In other words the terms \(a{n^2}/{V^2}\) and \(nb\) correct the pressure and volume in the ideal gas equation respectively.

Considering homogeneity of molecules of real gas in a container, intermolecular forces on the molecules towards the middle part of the container cancel but there is a net attracting force on the molecules at the adjacent layer of the walls of the container which reduce the pressure of gas on the walls. Therefore, the net attraction of molecules near the walls tends to decrease the pressure of real gas.