The osmotic pressure of a phenol solution in an organic solvent is determined to be 20% less than expected, it is due to

phenol  dissolved in organic solvent showed 20% lesser osmotic pressure than expected

I) phenol is 20% ionized ; can never be true

Because on ionization no.of particles increases and i>1

2) If phenol dimerises, n=2

For solute undergoing association in any solvent,

So  40% phenol  dimerises