Concept: Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom (typically F, O, or N). This creates a highly polarized bond, leaving the hydrogen atom with a significant partial positive charge and the electronegative atom with a significant partial negative charge. The partially positive hydrogen atom is then attracted to the lone pair of electrons on another highly electronegative atom in an adjacent molecule.

Why (B) is correct:

We need to identify molecules where hydrogen is directly bonded to Fluorine (F), Oxygen (O), or Nitrogen (N). Let's analyze each molecule:

• $C{H}_{3}F$ (Methyl fluoride): Hydrogen is bonded to carbon, not directly to fluorine. While fluorine is electronegative, the C-H bond is not sufficiently polarized to form strong hydrogen bonds.
• $C{H}_{3}OH$ (Methanol): Hydrogen is directly bonded to oxygen (O-H bond). This molecule can exhibit hydrogen bonding.
• $N{H}_3$ (Ammonia): Hydrogen is directly bonded to nitrogen (N-H bond). This molecule can exhibit hydrogen bonding.
• $H_{2}O$ (Water): Hydrogen is directly bonded to oxygen (O-H bond). This molecule can exhibit hydrogen bonding.
• $HF$ (Hydrogen fluoride): Hydrogen is directly bonded to fluorine (H-F bond). This molecule can exhibit hydrogen bonding.
• $C{H}_{3}OC{H}_3$ (Dimethyl ether): Oxygen is present, but hydrogen is bonded to carbon (C-H bonds). There are no O-H bonds. Therefore, it cannot exhibit hydrogen bonding with itself.
• $C{H}_{3}COOH$ (Acetic acid): Hydrogen is directly bonded to oxygen in the carboxyl group (O-H bond). This molecule can exhibit hydrogen bonding.
• $HCl$ (Hydrogen chloride): Hydrogen is bonded to chlorine. Although chlorine is electronegative, it is not electronegative enough (compared to F, O, N) to form strong hydrogen bonds.

The molecules that can exhibit hydrogen bonding are $C{H}_{3}OH$, $N{H}_3$, $H_{2}O$, $HF$, and $C{H}_{3}COOH$. There are 5 such molecules.

Option Analysis:

• A) 4: Incorrect, as 5 molecules can exhibit hydrogen bonding.
• B) 5: Correct, as $C{H}_{3}OH$, $N{H}_3$, $H_{2}O$, $HF$, and $C{H}_{3}COOH$ can form hydrogen bonds.
• C) 6: Incorrect, as only 5 molecules can exhibit hydrogen bonding.
• D) 7: Incorrect, as only 5 molecules can exhibit hydrogen bonding.

Correct Answer: (B)