Abnormal Molar Mass — Van't Hoff factor, association, dissociation
Solutions
10
JEE Qs
8%
Hard
60
min
Always first determine if the solute is undergoing dissociation, association, or neither, then correctly calculate the Van't Hoff factor 'i' before applying it to the colligative property formulas.
🧮 Key Formulas
✅ Key Points for JEE
- 1The Van't Hoff factor (i) quantifies the extent of dissociation or association of a solute, indicating how the number of particles in solution changes from initial expectations.
- 2For dissociation, i > 1 as the number of particles increases; for association, i < 1 as the number of particles decreases; for non-electrolytes, i = 1.
- 3Abnormal molar mass is observed when the calculated molar mass from colligative properties differs from the normal molar mass, which is a direct consequence of i \neq 1.
- 4Always identify if the solute is an electrolyte (dissociation) or tends to associate (e.g., carboxylic acids in non-polar solvents) to determine the applicability of 'i'.
- 5The degree of dissociation (\alpha) or association is critical for calculating 'i' when the process is not 100% complete (e.g., weak electrolytes).
⚠️ Common Mistakes
- ✕Incorrectly determining 'n' (number of particles formed) for dissociation, especially for complex salts or coordination compounds (e.g., K₄[Fe(CN)₆] gives 5 ions).
- ✕Confusing the formulas for degree of dissociation and degree of association, leading to sign errors in 'i' calculation.
- ✕Forgetting to multiply the Van't Hoff factor 'i' into the standard colligative property formulas, assuming ideal behavior.
- ✕Using molarity instead of molality (or vice versa) in colligative property calculations where specific concentration units are required.
📝 Practice Questions
See allQ73.Consider the following cases of standard enthalpy of reaction (ΔH∘r in kJmol−1) 7 C2H6( g) + O2( g) →2CO2( g) + 3H2O(l)ΔH∘1 = −1550 2 C (graphite) + O2( g) →CO2( g) ΔH∘2 = −393.5 The magnitude of ΔH∘fC2H6( g) is _______ 1 H2( g) + O2( g) →H2O(l) ΔH∘3 = −286 2 kJmol−1 (Nearest integer).
Q70.When a non-volatile solute is added to the solvent, the vapour pressure of the solvent decreases by 10 mm of Hg . The mole fraction of the solute in the solution is 0.2 . What would be the mole fraction of the solvent if decrease in vapour pressure is 20 mm of Hg ? (1) 0.8 (2) 0.4 (3) 0.2 (4) 0.6 Q71.0.01 mole of an organic compound (X) containing 10% hydrogen, on complete combustion produced 0.9 gH2O. Molar mass of (X) is _____ gmol−1 .
Q69.Arrange the following solutions in order of their increasing boiling points. (i) 10−4M NaCl (ii) 10−4M Urea (iii) 10−3M NaCl (iv) 10−2M NaCl (1) (i) < (ii) < (iii) < (iv) (2) (iv) < (iii) < (i) < (ii) (3) (ii) < (i ) ≡(iii) < (iv) (4) (ii) < (i) < (iii) < (iv)
Q52.Density of 3 M NaCl solution is 1.25 g/mL. The molality of the solution is : (1) 1.79 m (2) 2.79 m (3) 2 m (4) 3 m
Q64.Consider a binary solution of two volatile liquid components 1 and 2. x1 and y1 are the mole fractions of component 1 in liquid and vapour phase, respectively. The slope and intercept of the linear plot of 1 vs 1 x1 y1 are given respectively as: P02−P01 (1) P0 P02−P01 (2) P0 2 , 1 , P01 P02 P02 P02 P01−P02 (3) P0 P01−P02 (4) P0 1 , 2 , P02 P02 P01 P02
Q69.Consider the given plots of vapour pressure (VP) vs temperature(T/K). Which amongst the following options is correct graphical representation showing ΔTf , depression in the freezing point of a solvent in a solution? (1) (2) (3) (4)
NCERT Chapters
- Class 12 Chemistry Ch 2: Solutions