Phenol (mol.wt.94) dimerised to 60% in a solvent, the observed molar mass of phenol will be

$\mathrm{\alpha }=\frac{1-\mathrm{i}}{1-1/\mathrm{n}}$

$0.6=\frac{1-\mathrm{i}}{1-1/2}$

$\mathrm{i}=0.7$

$\mathrm{i}=\frac{\mathrm{Normal}\mathrm{molar}\mathrm{mass}}{\mathrm{abnormal}\mathrm{molar}\mathrm{mass}}$

Abnormal molar mass = $\frac{94}{0.7}=134.28$