I read it from a book but don't know the reason. Please explain...
Boiling point orders are -
Ethanamide > Ethanoic Anhydride > Ethanoic Acid
Ethanamide has a higher boiling point than ethanoic acid since amide has two hydogen atoms it can form more Hydrogen bonds than ethanoic acid and their molar masses are 45gmol-1 and 60gmol-1 which makes the difference brought by the London forces somewhat negligible . you may suppose Ethanoic anhydride should be having a lower boiling point than ethanoic acid and ethanamide since it only has London forces and dipole dipole attractions.But ehanoic anhydride(molar mass-106gmol-1) has a higher molar mass than Ethanoic acid(molar mass-60gmol-1) making the London forces in ehatnoic anhydride more significant. Hence we have ethanamide(222 °C)>Ethanoic anhydride(140°C)>Ethanoic acid(118°C).