According to Trouton's rule, the entropy of vaporization at the boiling point is approximately 85 J/kmol. This is because approximately the same increase in positional disorder occurs when any liquid is converted into vapor, so we can expect the change in entropy to be much the same in each case. Substantial departures from Trouton's rule normally indicate that the molecules have a more orderly arrangement, because of extensive intermolecular interactions.
9.17 is telling you to assume that the o
(vap) of CH3
is 85 J/Kmol in the equation
to get 253 K.