The properties which depend on the number of solute particles irrespective of their nature relative to the total numbers of particles in the solution are called colligative properties. These are relative lowering of vapour pressure of the solvent, depression of freezing point of the solvent, elevation of boiling point of the solvent and osmotic pressure of the solution. All of these properties help in calculating the observed molar mass of the solute. Determination of osmotic pressure is considered best for the determination of molecular mass of the solute.
Based on solute solvent interactions, arrange the following in order of increasing solubility in n-octane.
(A) Cycloheptane
(B) NaCl
(C) CH3OH
(D) CH3CN
Choose the correct answer from the options given below :
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B) < (C) < (D) < (A)
Solubility follows the rule “like dissolves like”: a solute dissolves well in a solvent when the forces between their molecules are similar. n-Octane is a non-polar solvent held only by dispersion forces, so the less polar the solute, the more soluble it is.
(B) NaCl — 1st (least soluble). An ionic solid; the weak dispersion forces with octane cannot make up for its strong lattice forces.
(C) \( \ce{CH3OH} \) — 2nd. Polar and hydrogen bonded; these strong interactions are lost on dissolving in octane, so it is poorly soluble.
(D) \( \ce{CH3CN} \) — 3rd. Polar (dipole-dipole forces) but not hydrogen bonded, so less is lost on mixing with octane than for methanol.
(A) Cycloheptane — 4th (most soluble). A non-polar hydrocarbon like octane; the forces in the solute and in the solvent are of the same kind.
Shortcut: NaCl must be least soluble and the hydrocarbon most soluble; Option (2) is the only one that starts with (B) and ends with (A).
NCERT (2026-27 edition):
Solubility of a Solid in a Liquid – “Every solid does not dissolve in a given liquid. While sodium chloride and sugar dissolve readily in water, naphthalene and anthracene do not. On the other hand, naphthalene and anthracene dissolve readily in benzene but sodium chloride and sugar do not. It is observed that polar solutes dissolve in polar solvents and non polar solutes in non-polar solvents. In general, a solute dissolves in a solvent if the intermolecular interactions are similar in the two or we may say like dissolves like.”