Solutions – Class 12 Chemistry notes, important questions and MCQs

Chapter overview

Solutions are homogeneous mixtures. Concentration terms, Raoult's law and colligative properties describe their behaviour.

Concentration and Raoult's law

• Molarity M = moles of solute / litres of solution; molality m = moles of solute / kg of solvent (temperature-independent).
• Mole fraction x = moles of a component / total moles.
• Raoult's law: p = p°x; ideal solutions obey it; non-ideal show positive or negative deviation (azeotropes).

Colligative properties

• Depend on the number of solute particles, not their nature.
• Relative lowering of vapour pressure = x_solute; ΔT_b = K_b m; ΔT_f = K_f m.
• Osmotic pressure π = CRT; van't Hoff factor i corrects for dissociation/association.

Key terms

Molality

moles of solute per kilogram of solvent

Osmosis

flow of solvent through a semipermeable membrane into a concentrated solution

Van't Hoff factor

i = observed colligative property / calculated value; accounts for association or dissociation

Formulas

• ΔT_f = i K_f m
• π = iCRT
• p_A = p°_A x_A

Important questions with answers

Quick revision

• Colligative = number of particles.
• NaCl gives i ≈ 2.
• Isotonic solutions have equal osmotic pressure.

Exam tips

• Use molality for boiling/freezing point problems because it does not change with temperature.
• Colligative properties depend only on the number of solute particles, so NaCl (i ≈ 2) has about double the effect of glucose at the same molality.

Practice MCQs

1. Molality is expressed as

   1. mol/L
   2. g/mol
   3. g/L
   4. mol/kg
   Show answer

   Answer: (d) mol/kg. Per kg of solvent.

2. Colligative properties depend on

   1. number of solute particles
   2. nature of solute
   3. colour
   4. density only
   Show answer

   Answer: (a) number of solute particles. Particle count.

3. van't Hoff factor for NaCl (complete dissociation) is

   1. 1
   2. 3
   3. 2
   4. 0.5
   Show answer

   Answer: (c) 2. Na⁺ + Cl⁻.

4. Which property is used to find molar masses of proteins?

   1. boiling point elevation
   2. vapour pressure
   3. freezing point depression
   4. osmotic pressure
   Show answer

   Answer: (d) osmotic pressure. Measurable at room temperature for small concentrations.

5. Which 0.1 m aqueous solution has the highest boiling point

   1. glucose
   2. NaCl
   3. urea
   4. CaCl₂
   Show answer

   Answer: (d) CaCl₂. i = 3.

6. 0.1 molal aqueous urea freezes at (K_f = 1.86 K kg mol⁻¹)

   1. −0.372 °C
   2. −1.86 °C
   3. −0.186 °C
   4. 0.186 °C
   Show answer

   Answer: (c) −0.186 °C. ΔT_f = 1.86 × 0.1.

7. A solution showing positive deviation from Raoult's law has

   1. stronger A–B attraction
   2. weaker A–B attraction than A–A and B–B
   3. zero mixing
   4. exact ideal behaviour
   Show answer

   Answer: (b) weaker A–B attraction than A–A and B–B. Vapour pressure higher than expected.

8. Osmotic pressure of 0.1 M glucose at 300 K (R = 0.0821 L atm/mol K) is about

   1. 2.46 atm
   2. 0.25 atm
   3. 24.6 atm
   4. 0.0246 atm
   Show answer

   Answer: (a) 2.46 atm. π = CRT.