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Solutions

Chemistry · Class 12NCERT pp. 1–272 sets × 25 questionsFree

Key facts and formulas for Solutions, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.

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Formula sheet for this chapter

IdeaRule or fact
Mass % and mole fractionMass % = (w solute / w solution) × 100; x₁ = n₁ / (n₁ + n₂)
Molarity and molalityM = moles / L of solution; m = moles / kg of solvent. Only molarity changes with temperature
Henry's lawx = p / K_H; solubility ∝ partial pressure; smaller K_H means more soluble
Raoult's lawp_A = p°_A x_A; p_total = p_A + p_B; y_B = p_B / p_total
DeviationsPositive: A–B weaker, min-boiling azeotrope. Negative: A–B stronger, max-boiling azeotrope
Boiling and freezingΔT_b = i K_b m; ΔT_f = i K_f m; M₂ = 1000 w₂ K / (ΔT w₁)
Osmotic pressureΠ = i C R T (T in kelvin); isotonic means equal Π; reverse osmosis needs pressure > Π
van't Hoff factori = observed / calculated effect; dissociation: i = 1 + (n − 1)x; association (dimer): i = 0.5
Mixing of ideal liquidsΔmix H = 0, Δmix V = 0, Δmix S > 0, Δmix G < 0

Three sample questions

Q1. Match the item with its type of solution:
A. Alloy &nbsp; B. Sodium amalgam &nbsp; C. Soda water &nbsp; D. Smoke
(i) gas in liquid (ii) solid in gas (iii) solid in solid (iv) liquid in solid
  1. A-iv, B-iii, C-i, D-ii
  2. A-iii, B-iv, C-i, D-ii
  3. A-iii, B-iv, C-ii, D-i
  4. A-i, B-ii, C-iii, D-iv
Hint and solution

Hint: Name the solvent (the bulk phase) and the solute for each item. In an amalgam the metal is the bulk phase.

  1. Type = state of solute in solvent — Decide which component is the solvent and which is the solute, then name the type as solute in solvent.
  2. Alloy = solid in solid; amalgam = liquid in solid — An alloy is a solid mixture of metals. Sodium amalgam is mercury, a liquid, dissolved in solid sodium.
  3. Soda water = gas in liquid; smoke = solid in gas — Soda water is carbon dioxide gas dissolved in water. Smoke is tiny solid particles spread in air.
  4. A-iii, B-iv, C-i, D-ii — So the match is A with solid in solid, B with liquid in solid, C with gas in liquid, D with solid in gas. Option B.
Answer: B. Alloys are solid in solid. Na–Hg amalgam is liquid in solid. Soda water is CO₂ (gas) in water (liquid). Smoke is solid particles in gas.
Q2. 15 g of sugar is dissolved in 135 g of water. The mass percentage of sugar is
  1. 11.1 %
  2. 15 %
  3. 9 %
  4. 10 %
Hint and solution

Hint: Mass percentage uses the mass of the whole solution in the denominator, not just the water.

  1. Mass % = (solute / solution) × 100 — Mass percentage compares the solute mass with the mass of the whole solution.
  2. Solution = 15 + 135 = 150 g — Add the sugar and the water to get the solution mass.
  3. Mass % = 15 / 150 × 100 = 10 % — Divide and multiply by 100. The sugar is 10 percent of the solution by mass. Option D.
Answer: D. Mass of solution = 150 g. Mass % = 15 / 150 × 100 = 10 %.
Q3. A solution contains 23 g ethanol (46 g/mol) and 54 g water (18 g/mol). The mole fraction of ethanol is
  1. 1/7
  2. 1/6
  3. 1/3
  4. 1/8
Hint and solution

Hint: Convert both masses to moles first, then divide moles of ethanol by total moles.

  1. n = mass / molar mass — Moles of a substance equal its mass divided by its molar mass.
  2. n(EtOH) = 23/46 = 0.5; n(H₂O) = 54/18 = 3 — Ethanol has 0.5 mol and water has 3 mol.
  3. x = 0.5 / (0.5 + 3) = 0.5 / 3.5 — Mole fraction is moles of ethanol divided by total moles.
  4. x = 1/7 ≈ 0.143 — Dividing gives one seventh. Option A.
Answer: A. Ethanol = 0.5 mol; water = 3 mol. x = 0.5 / 3.5 = 1/7.

How to use this chapter

Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.

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