Colligative Properties — ΔTb ΔTf Osmosis
Solutions
44
JEE Qs
10%
Hard
75
min
Master the correct application of the Van't Hoff factor (i) for all colligative properties, especially for electrolyte solutions with varying degrees of dissociation or association.
🧮 Key Formulas
✅ Key Points for JEE
- 1Colligative properties depend only on the number of solute particles, not their chemical nature.
- 2The Van't Hoff factor (i) is crucial for electrolyte solutions (salts, acids, bases) to account for dissociation or association of solute particles. Always consider 'i' unless explicitly stated that the solute is non-electrolyte.
- 3Molality (m) is used for ΔT_b and ΔT_f calculations, while Molarity (C) is used for osmotic pressure (π). Ensure correct units for R (gas constant) and T (absolute temperature in Kelvin) in the osmotic pressure formula.
- 4Abnormal molar masses are observed when solutes undergo dissociation (observed molar mass < normal) or association (observed molar mass > normal), directly related to the Van't Hoff factor 'i'.
- 5RLVP is often the first colligative property introduced, but ΔT_b, ΔT_f, and π are more commonly tested, especially with complex 'i' calculations.
⚠️ Common Mistakes
- ✕Forgetting to apply the Van't Hoff factor (i) for electrolyte solutions, or incorrectly calculating 'i' for partial dissociation/association.
- ✕Confusing molality (m) with molarity (C); using molarity instead of molality for ΔT_b and ΔT_f, or vice-versa for π.
- ✕Incorrectly using units for R (gas constant) and T (temperature) in the osmotic pressure equation (e.g., using °C instead of Kelvin, or using R in J/mol·K instead of L·atm/mol·K).
- ✕Not understanding that 'P°' in RLVP refers to vapor pressure of pure solvent, and 'P' to vapor pressure of solution.
📝 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 Part I Ch 2: Solutions