Test: Abnormal Molar Masses (NCERT) - NEET MCQ

Due to association or dissociation of solute molecules there is a change in number of particles. Since colligative properties depend on number of particles there is a change in molecular mass.

In case of dissociation: A_n → nA
Initial number of moles: 1  0
After dissociation:  1 − α nα
No. of particles = 1 − α + nα

K₄[Fe(CN)₆] → 4K⁺ + [Fe(CN)₆]^4-
Al₂​(SO₄​)_3​ → 2Al³⁺ + 3SO₄^2-

10⁻⁴M NaCl i = 2
10⁻⁴M Urea i = 1
10⁻³M MgCl₂ i = 3
10⁻²M NaCl i = 2
More the value of i, C, more will be the elevation in boiling point hence increasing order of boding point is 10⁻⁴M Urea < 10⁻⁴M NaCl < 10⁻³M MgCl₂ < 10⁻²M NaCl

C₆H₁₂O₆ is a non-electrolyte hence furnishes minimum number of particles and will have maximum freezing point.
ΔT_f = iK_fm or ΔT_f ∝ i and ΔT_f = T^∘_f − T_f

Urea is non-electrolyte, hence will not dissociate to give ions.

Mg(NO₃)₂ → Mg²⁺ + 2NO⁻₃

Degree of dissociation = 0.87 x 100 = 87%

For the same concentration of different solvents any colligative property ∝ = i
For NaCl, i = 2
Sugar solution, i = 1
BaCl₂, i = 3; FeCl₃, i = 4
Thus, for sugar solution depression in freezing point is minimum i.e., highest freezing point.

5A → A₅

ΔT_f = iK_f × m = 2 × 1.86 × 0.5 = 1.86^∘ C
T_f = T^∘_f − ΔT_f = 0 − 1.86 = −1.86^∘ C

Amount of CaCl₂ = n × M = 0.0164 × 111 = 1.820 g

KCl ⇔ K⁺ + Cl⁻  (n = 2)

MgCl₂ ⇔ Mg²⁺ + 2Cl⁻, i = 3