Below are 10 most repeated / high-probability board exam questions from Class 12 Chemistry – Chapter: Solutions, each with detailed answers.
(These are based on the latest NCERT pattern, past-year CBSE papers & sample papers.)
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⭐ 10 Most Repeated Board Questions – Solutions (Class 12 Chemistry)
With Detailed Answers
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1. Define molality and molarity. Why is molality preferred over molarity?
Answer:
Molarity (M):
Number of moles of solute dissolved in 1 litre of solution.
M = \frac{\text{moles of solute}}{\text{volume of solution (L)}}
Molality (m):
Number of moles of solute dissolved in 1 kg of solvent.
m = \frac{\text{moles of solute}}{\text{mass of solvent (kg)}}
Why molality is preferred?
Molality is temperature independent, because it depends on mass (kg).
Molarity changes with temperature because volume changes with temperature.
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2. State Raoult’s Law for a solution of volatile liquids.
Answer:
For a solution containing two volatile components A and B:
p_A = x_A p_A^\circ,\quad p_B = x_B p_B^\circ
Total vapour pressure:
p = p_A + p_B
Where:
= vapour pressures of pure components
= mole fractions
Meaning:
Each component contributes to total vapour pressure in proportion to its mole fraction.
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3. What are ideal and non-ideal solutions? Give examples.
Answer:
Ideal solution
A solution that follows Raoult’s law at all compositions and temperatures.
Characteristics:
No enthalpy change:
No volume change:
Intermolecular forces A–A, B–B, A–B are equal.
Examples: benzene + toluene, n-hexane + n-heptane
Non-ideal solution
A solution that does NOT follow Raoult’s law and shows deviation.
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4. Explain positive and negative deviations from Raoult’s Law. Give two examples.
Answer:
Positive deviation
A–B attraction < A–A or B–B
High vapour pressure
Endothermic mixing
Examples: Ethanol + acetone, ethanol + water
Negative deviation
A–B attraction > A–A or B–B
Low vapour pressure
Exothermic mixing
Examples: Acetone + chloroform, HNO₃ + water
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5. Define azeotropes. Write the types with examples.
Answer:
Azeotropes are constant-boiling mixtures whose composition cannot be changed by distillation.
Types:
1. Minimum boiling azeotrope (positive deviation)
Example: Ethanol–water (95% ethanol)
2. Maximum boiling azeotrope (negative deviation)
Example: HCl–water (20% HCl)
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6. What is Henry’s Law? Write its formula and application.
Answer:
Henry's law states:
"The solubility of a gas in a liquid is directly proportional to the partial pressure of the gas."
p = k_H x
Where:
= pressure
= mole fraction of gas
= Henry’s law constant
Applications:
Carbonated drinks
Deep-sea diving gases
Purification of water
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7. What is colligative property? Name four colligative properties.
Answer:
Colligative properties depend only on number of solute particles, not on their nature.
Four colligative properties:
1. Relative lowering of vapour pressure
2. Elevation in boiling point
3. Depression in freezing point
4. Osmotic pressure
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8. Derive the relation between elevation in boiling point and molality.
Answer:
Elevation in boiling point:
\Delta T_b = K_b m
Where:
= molal elevation constant
= molality
Derivation idea:
Raoult’s law → lowering of vapour pressure → Clausius–Clapeyron equation → relation between ΔTb and molality.
Final formula:
\Delta T_b = K_b m
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9. Write the Van’t Hoff factor (i) and its significance.
Answer:
Van’t Hoff factor (i):
i = \frac{\text{observed colligative property}}{\text{normal colligative property}}
Significance:
Accounts for association (i < 1)
Accounts for dissociation (i > 1)
Example:
NaCl → Na⁺ + Cl⁻ → i ≈ 2
CH₃COOH (association in benzene) → i < 1
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10. Define osmotic pressure. Derive the formula used to calculate molar mass.
Answer:
Osmotic pressure (π):
\pi = MRT
Where:
= molarity of solution
= gas constant
= temperature in Kelvin
Relation for molar mass:
\pi = \frac{n}{V} RT = \frac{w}{M_2} \cdot \frac{RT}{V}
So,
M_2 = \frac{wRT}{\pi V}
This formula is used to determine molar mass of solutes (especially polymers).
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If you want, I can also prepare:
📘 Top 20 very important numericals
📝 Full chapter mock test (40 marks)
📄 Board-exam style case-based questions for Solutions
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