Class 12 Chemistry chapter “Solutions”, designed based on previous year questions (PYQs)

 Class 12 Chemistry chapter “Solutions”, designed based on previous year questions (PYQs) from CBSE board exams. Each question includes a concise answer and covers important concepts like concentration terms, Raoult’s Law, colligative properties, abnormal molar mass, and more.

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📘 Chapter: Solutions – Class 12

🧪 PYQ-Based Questions with Answers

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1. Define mole fraction. What is the sum of mole fractions of all components in a solution?

Answer:
Mole fraction is the ratio of moles of one component to the total moles of all components in the solution.

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2. State Henry’s Law and mention its application.

Answer:
Henry’s Law: The solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid.

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3. What is the effect of temperature on solubility of gases in liquids?

Answer:
The solubility of gases in liquids decreases with increase in temperature because dissolution of gas is an exothermic process.

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4. Define ideal and non-ideal solutions. Give one example of each.

Answer:

• Ideal solution: Obeys Raoult’s law at all concentrations (e.g., benzene + toluene).
• Non-ideal solution: Shows deviation from Raoult’s law (e.g., acetone + chloroform).

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5. State Raoult’s Law for a solution of volatile liquids.

Answer:
Raoult’s Law: The partial vapor pressure of each component in a solution is directly proportional to its mole fraction.

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6. A solution contains 30 g of urea (M = 60 g/mol) in 246 g of water. Calculate the mole fraction of urea.

Given:

• Mass of urea = $30 \, \text{g}$

• Molar mass of urea (M) = $60 \, \text{g mol}^{-1}$

• Mass of water = $246 \, \text{g}$

• Molar mass of water = $18 \, \text{g mol}^{-1}$

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Step 1: Calculate moles of urea

$$n_{\text{urea}}=\frac{\text{mass}}{\text{molar mass}}=\frac{30}{60}=0.5\text{mol}$$

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Step 2: Calculate moles of water

$$n_{\text{water}} = \frac{246}{18} = 13.67 \, \text{mol (approx.)}$$

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Step 3: Calculate mole fraction of urea

Mole fraction of urea:

$$X_{\text{urea}} = \frac{n_{\text{urea}}}{n_{\text{urea}} + n_{\text{water}}} = \frac{0.5}{0.5 + 13.67}$$ $$X_{\text{urea}} = \frac{0.5}{14.17} \approx 0.035$$

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✅ Final Answer:
The mole fraction of urea is 0.035 (approx.)

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7. What is meant by abnormal molar mass? How is it caused?

Answer:
Abnormal molar mass is the molar mass calculated using colligative properties that differs from the actual molar mass due to association or dissociation of solute particles in solution.

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8. A solution of glucose (C₆H₁₂O₆) in water is 1 molal. What is the freezing point of the solution? (Kf for water = 1.86 K kg/mol).

Given:

• Molality ($m$) = $1 \, \text{mol/kg}$
  

• Solute = glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) → a non-electrolyte (does not dissociate, so $i = 1$)

• $K_f$ for water = $1.86 \, \text{K·kg/mol}$
  

• Normal freezing point of water = $0^\circ \text{C}$

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Step 1: Formula for depression in freezing point

$$\Delta T_f = i \cdot K_f \cdot m$$

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Step 2: Substitute values

$$\Delta T_f = 1 \times 1.86 \times 1 = 1.86 \, \text{K}$$

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Step 3: Calculate new freezing point

$$T_f = 0^\circ \text{C} - \Delta T_f = 0 - 1.86 = -1.86^\circ \text{C}$$

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✅ Final Answer:
The freezing point of the solution is –1.86 °C.

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9. What is van’t Hoff factor (i)? What is its value for glucose and NaCl in aqueous solution?

Case 1: Glucose (C₆H₁₂O₆)

• Glucose is a non-electrolyte.

• It does not ionize in water.

• Each molecule stays intact.

$$i=1$$

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Case 2: Sodium Chloride (NaCl)

• NaCl is an electrolyte.

• It dissociates completely in aqueous solution:

$$\text{NaCl}⟶{\text{Na}}^{+}+{\text{Cl}}^{-}$$

• 1 formula unit of NaCl → 2 particles.

$$i=2$$

(Note: In practice, due to incomplete dissociation and ion pairing, the experimental value may be slightly less than 2.)

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✅ Final Answer:

• For glucose: $i = 1$
  

• For NaCl: $i \approx 2$
  

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10. What are azeotropes? Give one example each of minimum and maximum boiling azeotrope.

Answer:
Azeotropes are mixtures that boil at a constant temperature without change in composition.

• Minimum boiling: Ethanol + water
• Maximum boiling: HNO₃ + H₂O