Arrange the following in the increasing order of their van’t Hoff factors:
(A) Very dilute \(MgCl_2\) solution
(B) Very dilute \(AlCl_3\) solution
(C) Very dilute \(NaCl\) solution
(D) Very dilute \(Al_2(SO_4)_3\) solution
(E) Very dilute Urea solution
Choose the correct answers from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (E) < (C) < (A) < (B) < (D)
Note: NTA dropped this question in the official Answer Key. However, to provide additional question practice to the students, the question has been slightly modified so that the question has one clear answer.
The van’t Hoff factor i is the number of particles one formula unit gives in solution. In a very dilute solution a strong electrolyte dissociates completely, so i equals the number of ions in the formula; a non-electrolyte gives one particle.
(E) Urea does not dissociate: 1 particle, i = 1.
(C) NaCl → Na+ + Cl−: 1 + 1 = 2 ions, i = 2.
(A) MgCl2 → Mg2+ + 2Cl−: 1 + 2 = 3 ions, i = 3.
(B) AlCl3 → Al3+ + 3Cl−: 1 + 3 = 4 ions, i = 4.
(D) Al2(SO4)3 → 2Al3+ + 3SO42−: 2 + 3 = 5 ions, i = 5.
Shortcut: every option starts with urea. Only Options (1), (2) and (3) put NaCl second, and of these only Option (1) has \( \ce{AlCl3} \) (4 ions) after \( \ce{MgCl2} \) (3 ions) and \( \ce{Al2(SO4)3} \) (5 ions) last.
NCERT (2026-27 edition):
Abnormal Molar Masses – …
