Van't Hoff factor \((i)\) is given by
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Total number of moles of particles after dissociation / Total number of moles of particles before dissociation
The van’t Hoff factor (i) accounts for association or dissociation of the solute. Option (3) is the third of the book’s three definitions of i. For example, NaCl dissociating completely gives 2 mol of ions from 1 mol, so i = 2.
Why the other options are incorrect
Option (1) Abnormal molar mass / Normal molar mass — Incorrect. Abnormal molar mass / normal molar mass is the reciprocal of i. For dissociation the abnormal (observed) molar mass is smaller, so this ratio would be less than 1 when i is actually greater than 1.
Option (2) Calculated colligative property / Observed colligative property — Incorrect. Calculated / observed colligative property is also the inverse of i; i is observed divided by calculated.
Option (4) Total number of moles of particles before dissociation / Total number of moles of particles after dissociation — Incorrect. Moles before / moles after dissociation is the reciprocal of the correct ratio.
NCERT (2026-27 edition):
Abnormal Molar Masses – …
