Experimentally determined molar mass is always lower than the true value in case of :
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Strong electrolyte
Colligative properties depend on the number of solute particles. A molar mass worked out from a colligative property assumes that the solute neither dissociates nor associates. A strong electrolyte dissociates completely into ions (e.g., KCl → \( \ce{K^{+}} \) + \( \ce{Cl^{-}} \)), so the number of particles increases, the colligative property is larger than expected and the experimentally determined (abnormal) molar mass is always lower than the true value (i > 1).
Why the other options are incorrect
Option (2) Weak electrolyte — Incorrect. A weak electrolyte dissociates only slightly, so its molar mass comes out only a little lower; and a weak acid such as ethanoic acid in benzene even dimerises and gives a higher value, so it is not “always” lower.
Option (3) Non-electrolyte — Incorrect. A non-electrolyte neither dissociates nor associates (i = 1), so its experimental molar mass equals the true value.
Option (4) Dimers — Incorrect. Association into dimers halves the number of particles, so the experimental molar mass is higher than the true value (i < 1).
NCERT (2026-27 edition):
Abnormal Molar Masses – “… In 1880 van’t Hoff introduced a factor i, known as the van’t Hoff factor, to account for the extent of dissociation or association. This factor i is defined as:
i = Normal molar mass / Abnormal molar mass
= Observed colligative property / Calculated colligative property
= Total number of moles of particles after association/dissociation / Number of moles of particles before association/dissociation
Here abnormal molar mass is the experimentally determined molar mass and calculated colligative properties are obtained by assuming that the non-volatile solute is neither associated nor dissociated.”