If the boiling point of ethanol (molecular weight = 46) is 78°C, the boiling point of diethyl ether (molecular weight = 74) is

There is hydrogen bonding between the ethanol molecules and thus, boiling point increases. But in case of diethyl ether, both side of O atom is surrounded by ethanol molecules. This decreases the polarity of molecules and intensity of formation of hydrogen bonding. Hence, boiling point of diethyl ether is lower than ethanol.