Solutions

What you'll learn

A friendly, NCERT-aligned walkthrough of the Solutions chapter: how to measure concentration (molarity, molality, mole fraction), what Raoult's Law says about vapour pressure, and how the four colligative properties let us find molar masses, with two fully worked numericals.

This chapter has

Solutions — solved practice questions

8 Class 12 Chemistry questions with step-by-step solutions. Attempt each, then reveal the answer.

1. Q1easy

   Which of the following is an example of a solid solution?

   • ASoda water
   • BBrass (copper and zinc)
   • CEthanol in water
   • DAerated water
   Show answer & solution

   Correct answer: (B) Brass (copper and zinc)

   Brass is an alloy of copper and zinc; both the solvent and solute are solids, so it is a solid solution. Soda water and aerated water are gas-in-liquid, while ethanol in water is liquid-in-liquid.

2. Q2easy

   Molarity of a solution is defined as the number of moles of solute dissolved in:

   • A1 kg of solvent
   • B1 litre of solvent
   • C1 kg of solution
   • D1 litre of solution
   Show answer & solution

   Correct answer: (D) 1 litre of solution

   Molarity (M) = moles of solute per litre of solution. In contrast, molality is moles of solute per kilogram of solvent.

3. Q3medium

   Which colligative property is used to determine the molar mass of a polymer or protein because it gives a measurable value even at low concentration?

   • ARelative lowering of vapour pressure
   • BElevation of boiling point
   • COsmotic pressure
   • DDepression of freezing point
   Show answer & solution

   Correct answer: (C) Osmotic pressure

   Osmotic pressure is measurable even at very low concentrations and near room temperature, making it ideal for macromolecules like polymers and proteins whose small molar concentrations give tiny changes in the other properties.

4. Q4easy

   According to Henry's law, the solubility of a gas in a liquid at a given temperature is:

   • AInversely proportional to its partial pressure
   • BIndependent of pressure
   • CProportional to the square of pressure
   • DDirectly proportional to its partial pressure
   Show answer & solution

   Correct answer: (D) Directly proportional to its partial pressure

   Henry's law states p = KH x, i.e. the partial pressure of a gas over the solution is proportional to its mole fraction, so solubility increases directly with partial pressure.

5. Q5medium

   A solution that shows positive deviation from Raoult's law will have:

   • ADelta H mixing negative and Delta V mixing negative
   • BDelta H mixing positive and Delta V mixing positive
   • CDelta H mixing zero and Delta V mixing zero
   • DDelta H mixing negative and Delta V mixing positive
   Show answer & solution

   Correct answer: (B) Delta H mixing positive and Delta V mixing positive

   Positive deviation occurs when A-B interactions are weaker than A-A and B-B, so mixing is endothermic (Delta H > 0) and the volume expands (Delta V > 0). Vapour pressure is higher than predicted.

6. Q6medium

   The van't Hoff factor (i) for a strong electrolyte like NaCl that dissociates completely in dilute solution is approximately:

   • A1
   • B0.5
   • C2
   • D3
   Show answer & solution

   Correct answer: (C) 2

   NaCl dissociates into Na+ and Cl- giving 2 particles per formula unit, so i is approximately 2 for complete dissociation in a dilute solution.

7. Q7medium

   Calculate the mole fraction of ethanol (molar mass 46 g/mol) in a solution containing 46 g of ethanol dissolved in 90 g of water (molar mass 18 g/mol).

   • A0.167
   • B0.500
   • C0.333
   • D0.833
   Show answer & solution

   Correct answer: (A) 0.167

   Moles of ethanol = 46/46 = 1; moles of water = 90/18 = 5. Mole fraction of ethanol = 1/(1+5) = 1/6 = 0.167.

8. Q8hard

   Which of the following aqueous solutions, each of 0.1 m concentration, will have the highest boiling point?

   • A0.1 m glucose
   • B0.1 m NaCl
   • C0.1 m BaCl2
   • D0.1 m urea
   Show answer & solution

   Correct answer: (C) 0.1 m BaCl2

   Elevation of boiling point depends on the number of particles (i x m). BaCl2 gives i approximately 3 (Ba2+ + 2Cl-), the most particles, so it has the greatest elevation and the highest boiling point.

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