Q.Arrange ethanol, propane and chloroethane, all of comparable molar mass, in increasing order of boiling point, and explain the order using intermolecular forces.

Propane is non-polar and held together only by weak London dispersion forces, giving it the lowest boiling point ($-42\,°\text{C}$). Chloroethane is polar (a permanent $\text{C--Cl}$ dipole) and so has somewhat stronger dipole-dipole attractions on top of dispersion forces, raising its boiling point to $12\,°\text{C}$, but it still cannot hydrogen-bond, since chlorine attached to carbon is not a hydrogen-bond donor/acceptor pair in the same way as $-\text{OH}$. Ethanol's $-\text{OH}$ group can both donate and accept hydrogen bonds, so ethanol molecules associate strongly with one another, giving by far the highest boiling point of the three ($78\,°\text{C}$) even though it is not the heaviest of the three molecules.

Rank the compounds by the strongest intermolecular force each can access: dispersion only, then dispersion+dipole, then dispersion+dipole+hydrogen bonding.

Assuming boiling point tracks molar mass alone, which would wrongly rank chloroethane (the heaviest) above ethanol.