Solutions — Class 12 MCQs with Answers

Solutions — important questions & MCQs with answers (Class 12 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Molarity is defined as:

   AMoles of solute per kg of solventBMoles of solute per litre of solutionCMass of solute per litre of solventDMass of solute per kg of solution
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   Answer: Moles of solute per litre of solution

   Explanation: Molarity = moles of solute ÷ litres of SOLUTION — that volume basis is what separates it from molality (mass of solvent) and rules out a kg-of-solvent style answer.

2. Q2 of 6Easy

   Raoult's law states that the partial vapour pressure of each component is proportional to its:

   AMolar massBMass fraction in solutionCMole fraction in solutionDBoiling point
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   Answer: Mole fraction in solution

   Explanation: Raoult's law: P_A = x_A·P°_A — partial vapour pressure scales with MOLE fraction, not mass fraction or molar mass, because vapour composition tracks particle count.

3. Q3 of 6Medium

   The van't Hoff factor (i) for NaCl in dilute aqueous solution (assuming complete dissociation) is approximately:

   A0.5B1C3D2
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   Answer: 2

   Explanation: NaCl → Na⁺ + Cl⁻ splits into 2 ions per formula unit, so the van't Hoff factor i ≈ 2 for a dilute solution near-complete dissociation — not i = 1, which would apply to a non-electrolyte like glucose.

4. Q4 of 6Medium

   Assertion (A): Climbers at high altitudes may become weak and unable to think clearly. Reason (R): At high altitudes the partial pressure of oxygen is lower than at ground level, so the concentration of oxygen in the blood and tissues becomes low.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: A is true: this condition is called anoxia. R is true and explains it: by Henry's law, a lower partial pressure of oxygen means less oxygen dissolves in the blood, and low blood oxygen causes the weakness and poor thinking.

5. Q5 of 6Hard

   A minimum-boiling azeotrope is formed by a solution showing:

   ANegative deviation from Raoult's lawBPositive deviation from Raoult's lawCIdeal behaviourDNo vapour-liquid equilibrium
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   Answer: Positive deviation from Raoult's law

   Explanation: Positive deviation raises the solution's vapour pressure above the ideal line, and a higher vapour pressure means the mixture boils at a LOWER temperature — giving a minimum-boiling azeotrope, e.g. ethanol + water.

6. Q6 of 6Hard

   Assertion (A): People who take a lot of salt or salty food may develop puffiness or swelling in their tissues. Reason (R): Commercial bleaching solution contains about 3.62 mass percentage of sodium hypochlorite in water.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true but R is not the correct explanation of A

   Explanation: A is true: high salt intake causes water to be retained in tissue cells and intercellular spaces by osmosis, giving edema. R is also true, but the composition of bleaching solution has nothing to do with water retention caused by salt.

Solutions notes: key concepts (Class 12 Chemistry)

Solutions builds every quantitative tool for liquid mixtures: concentration terms, Henry's law for gas solubility, Raoult's law with its positive and negative deviations that give azeotropes, and the four colligative properties. The van't Hoff factor i corrects for association or dissociation and explains abnormal molar masses.

Molarity vs molality

Molarity = mol solute per litre of SOLUTION (changes with temperature); molality = mol solute per kg of SOLVENT (temperature-independent, so used in ΔTb and ΔTf).

Mole fraction

Ratio of moles of one component to total moles; dimensionless and temperature-independent. Sum of all mole fractions in a solution equals 1.

Henry's law

Solubility of a gas in a liquid is proportional to its partial pressure: p = K_H·x. A higher Henry constant K_H means lower solubility of that gas.

Raoult's law

For an ideal solution, partial vapour pressure of each volatile component equals its mole fraction times its pure vapour pressure: p_A = x_A·p°_A.

Ideal solutions

Obey Raoult's law over the whole range; ΔH(mix)=0 and ΔV(mix)=0 because A–B interactions equal A–A and B–B, e.g. benzene + toluene.

Positive deviation

A–B forces weaker than A–A/B–B, so vapour pressure is higher than Raoult predicts; ΔH(mix)>0. Gives a minimum-boiling azeotrope (ethanol + water).

Negative deviation

A–B forces stronger than A–A/B–B, vapour pressure lower than predicted; ΔH(mix)<0. Gives a maximum-boiling azeotrope (nitric acid + water).

Azeotropes

Constant-boiling mixtures whose vapour and liquid have the same composition, so they cannot be separated by fractional distillation.

Relative lowering of vapour pressure

Adding a non-volatile solute lowers vapour pressure; (p°−p)/p° equals the mole fraction of the solute — a colligative property.

Elevation of boiling point

A non-volatile solute raises the boiling point by ΔTb = i·Kb·m, where Kb is the molal boiling-point elevation (ebullioscopic) constant of the solvent.

Depression of freezing point

A solute lowers the freezing point by ΔTf = i·Kf·m; the basis of using salt to melt ice and antifreeze in radiators.

Osmotic pressure

Pressure needed to stop solvent flow across a semipermeable membrane: Π = i·CRT. Being measurable at low concentration, it is best for finding molar mass of polymers/proteins.

van't Hoff factor

i = observed colligative effect / normal effect. i>1 for dissociation (NaCl≈2), i<1 for association (dimeric acids in benzene), i=1 for non-electrolytes.

Abnormal molar mass

When a solute associates or dissociates, the molar mass calculated from a colligative property differs from the true value; i is the correction factor.