Identify the most volatile compound

The correct answer is Option (4) → Propane

A volatile compound evaporates easily, that is, it has a low boiling point. The boiling point rises with the strength of the forces between molecules: hydrogen bonding is the strongest of these, and van der Waals forces grow with molar mass.

• n-Butanol, \( \ce{C4H9OH} \): strong O–H hydrogen bonds; b.p. about 391 K.

• Triethylamine, \( \ce{(C2H5)3N} \): a tertiary amine with no N–H hydrogen bonding, but a large molecule (101 g mol⁻¹); b.p. about 362 K.

• Propylamine, \( \ce{C3H7NH2} \): a primary amine with N–H hydrogen bonds; b.p. about 321 K.

• Propane, \( \ce{C3H8} \): a small non-polar molecule held only by weak van der Waals forces; b.p. about 231 K, a gas at room temperature.

So propane is the most volatile.

Why the other options are incorrect

• Option (1) n-butanol — Incorrect. Alcohols form the strongest hydrogen bonds here, so n-butanol is the least volatile.

• Option (2) Triethylamine — Incorrect. It has no hydrogen bonding, but its larger size gives strong van der Waals forces, so it boils far above propane.

• Option (3) Propylamine — Incorrect. Its N–H hydrogen bonds hold the molecules together, so it is a liquid at room temperature.