A sample of benzene which is contaminated with non-volatile substances boils at \(3^{\circ} \mathrm{C}\) higher than that of a sample of pure benzene. The normal boiling point of benzene is \(80.1^{\circ} \mathrm{C}\) and the latent heat of vaporization is \(30.76 \mathrm{~kJ} / \mathrm{mol}\). Estimate the mol per cent of the contaminants in the sample of benzene.