Final answer:
Acetic acid molecules form dimers option (b) through hydrogen bonds, where hydrogen from the hydroxyl group of one molecule bonds to the oxygen of the carboxyl group of another molecule. This is a type of intermolecular force and is common in carboxylic acids in the gas phase.
Step-by-step explanation:
Under certain conditions, molecules of acetic acid, CH3COOH, can form dimers in the gas phase, which are pairs of acetic acid molecules held together by hydrogen bonds.
A dimer of acetic acid consists of two acetic acid molecules linked such that the hydrogen atom from the hydroxyl group (OH) of one molecule forms a hydrogen bond (represented as a dashed line) with the oxygen atom of the carboxyl group (COOH) from another molecule.
This interaction is a type of intermolecular force (IMF) specifically called a hydrogen bond, which is characterized by the attraction between a hydrogen atom, which is covalently bonded to a more electronegative atom (like oxygen), and another electronegative atom from a different molecule.
The typical structure for an acetic acid dimer would be represented as two CH3COOH molecules with the hydrogen from one OH group forming a dashed line to the oxygen of the COOH group from the adjacent molecule, indicating the hydrogen bonding. It can be depicted as follows:
O-H----O This shows the O-H group of one molecule of acetic acid hydrogen-bonded to the oxygen of the carbonyl group (C=O) of another acetic acid molecule, forming the dimer.