Class 12 Chemistry, Chapter 1 (NCERT). 15 questions by concept. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
A solution is labelled 12% (w/w) glucose. Which interpretation is correct?
(b) 12 g glucose is present in 100 g solution.
Mass percentage uses mass of component divided by total mass of solution, multiplied by 100.
What does 5% (w/v) NaCl mean?
(a) 5 g NaCl is present in 100 mL solution.
Mass-by-volume percentage specifies grams of solute per 100 mL of final solution.
Powdered NH4Cl dissolves faster than equal-mass crystals at the same temperature. What follows about equilibrium solubility?
(c) Both forms have the same equilibrium solubility.
Particle size changes dissolution rate through surface area, not the equilibrium solubility at fixed temperature.
A soluble solid is added to water at fixed temperature and pressure. While some solid is still present, stirring increases mixing near its surface and makes it dissolve faster. Which property remains unchanged?
(d) The equilibrium solubility at that temperature.
Dissolution rate describes how quickly solid enters the solution. Stirring improves contact and transport near the solid surface, so the rate can increase.
Equilibrium solubility describes the maximum amount that can remain dissolved in a specified amount of solvent at the stated temperature and pressure. Stirring alone does not change that limit.
The unchanged property is equilibrium solubility. Faster dissolution does not mean a greater equilibrium solubility.
A non-volatile solute is added to a volatile solvent at fixed temperature. Which statement is correct?
(d) The solvent vapour pressure falls, and the vapour contains only solvent.
A non-volatile solute contributes no vapour and lowers the solvent mole fraction; the equilibrium vapour is therefore solvent while its pressure is reduced.
A volatile binary liquid has unequal pure-component vapour pressures. At equilibrium, what distinguishes vapour from liquid?
(b) The vapour is enriched in the more volatile component.
Unequal escaping tendencies make the vapour richer in the component with the greater pure vapour pressure.
A solution shows a large positive deviation from Raoult's law. Which statement is correct?
(d) It can form a minimum-boiling azeotrope that ordinary fractional distillation cannot cross.
Positive deviation raises vapour pressure and can create a boiling minimum; at the azeotrope liquid and vapour compositions coincide.
Why can an azeotrope not be separated into pure components by ordinary fractional distillation?
(d) Its liquid and equilibrium vapour have the same composition.
Distillation enriches a vapour only when vapour and liquid compositions differ; at an azeotrope they coincide.
Which method correctly compares ideal colligative effects in the same solvent?
(b) Compare effective particle molality, i m.
A colligative effect is governed by dissolved-particle concentration; solvent mass remains the molality denominator.
Two solutions in the same solvent have equal effective particle molality. Which conclusion is valid?
(a) They have equal ideal magnitudes of the same colligative property.
Once effective particle concentration is equal in the same solvent, identity does not change the ideal colligative magnitude.
Which statement about the molal freezing-point depression constant Kf is correct?
(d) Kf is a property of the solvent and multiplies molality.
Kf sets the depression per unit molality for a given solvent; concentration changes ΔTf, not Kf.
Which setup correctly uses ΔTf=Kfm?
(d) Use moles of solute per kilogram of solvent, with no 273.15 correction to ΔTf.
The symbol m denotes molality, and kelvin and Celsius temperature intervals have the same numerical magnitude.
One mole NaCl is dissolved in 1.000 kg water and dissociates completely. With water's Kb=0.52 K kg mol^-1, what boiling point is predicted?
(c) 101.04 °C
Complete NaCl dissociation gives i=2, so ΔTb=2×0.52×1.0=1.04 K and Tb=101.04 °C.
A red blood cell is placed in an isotonic solution. Which statement is correct?
(c) There is no net water flow, so the cell retains its normal volume.
Isotonic solutions have equal osmotic pressure at the stated temperature, so opposing solvent flows balance.
A solute undergoes complete tetramerisation. What is its ideal van't Hoff factor?
(a) 0.25 (dimensionless)
Four monomers become one particle, so particles after/before=1/4=0.25.
Wrong
The molarity remains unchanged because the amount of solute stays the same.
Why it feels right. I track the numerator and overlook the volume.
Right
Molarity uses solution volume. Molality uses solvent mass.
Worked example
| Sample | Volume | Molarity | Molality |
|---|---|---|---|
| Before | 1.00 L | 1.00 mol/L | 1.00 mol/kg |
| After | 1.02 L | 0.98 mol/L | 1.00 mol/kg |
