Chapter 1: Solutions Short Questions | chemistry Class 12 CBSE

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Chapter 1: Solutions Short Questions | chemistry Class 12 CBSE | ByteNotes.in

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Question 1

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Define a solution and state the role of solvent in determining its properties.

A solution is a homogeneous mixture of two or more components with uniform composition and properties throughout.
The solvent is the component present in the largest quantity and it determines the physical state in which the solution exists.
One or more other components present in the solution, called solutes, are dissolved in the solvent.

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Question 2

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Why is molality preferred over molarity when expressing concentration for experiments involving temperature changes?

Molality is defined as moles of solute per kilogram of solvent and is therefore independent of temperature.
Molarity depends on the volume of solution, which changes with temperature because liquids expand or contract on heating or cooling.
Hence, for experiments conducted at varying temperatures, molality gives a more consistent and reliable measure of concentration.

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Question 3

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State Henry's law for gas solubility and write its mathematical expression.

Henry's law states that at constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of the liquid.
Mathematically, p = KH × x, where p is the partial pressure of the gas, x is its mole fraction in solution, and KH is Henry's law constant.
A higher KH value at a given pressure indicates lower solubility of the gas in that liquid.

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Question 4

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Explain why the solubility of gases in liquids decreases with an increase in temperature.

When a gas dissolves in a liquid, the process is similar to condensation and heat is evolved, making dissolution an exothermic process.
According to Le Chatelier's Principle, for an exothermic equilibrium, increasing temperature shifts the equilibrium in the reverse (endothermic) direction.
This means the dissolved gas tends to escape back into the vapour phase, thereby decreasing the solubility of the gas with rising temperature.

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Question 5

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Define mole fraction and state the relationship between the mole fractions of all components in a solution.

Mole fraction of a component is defined as the ratio of the number of moles of that component to the total number of moles of all components in the solution.
For component i: xi = ni / (n1 + n2 + ... + ni).
The sum of mole fractions of all components in a given solution is always equal to unity: x1 + x2 + ... + xi = 1.

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Question 6

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What are ideal solutions? Give two examples from the chapter.

Ideal solutions are those that obey Raoult's law over the entire range of concentration, with enthalpy of mixing (ΔmixH) equal to zero and volume of mixing (ΔmixV) equal to zero.
At the molecular level, ideal behaviour arises when A–A, B–B, and A–B intermolecular interactions are nearly equal in magnitude.
Examples of nearly ideal solutions include n-hexane and n-heptane, and benzene and toluene.

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Question 7

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How does the addition of a non-volatile solute affect the vapour pressure of a solvent? Give the molecular reason.

When a non-volatile solute is added to a solvent, the vapour pressure of the solution is lower than that of the pure solvent at the same temperature.
This happens because solute molecules occupy part of the liquid surface, reducing the fraction of solvent molecules at the surface that can escape into the vapour phase.
The degree of lowering depends on the quantity of solute particles irrespective of their chemical nature, making it a colligative property.

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Question 8

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Define elevation of boiling point and write the expression relating it to molality of the solution.

Elevation of boiling point (ΔTb) is the increase in the boiling point of a solution compared to that of the pure solvent, caused by the addition of a non-volatile solute.
For dilute solutions, ΔTb is directly proportional to the molality (m) of the solute: ΔTb = Kb × m, where Kb is the molal elevation constant (ebullioscopic constant).
Kb is characteristic of the solvent and has units of K kg mol⁻¹; for water, Kb = 0.52 K kg mol⁻¹.

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Question 9

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Define depression of freezing point and state how it can be used to determine the molar mass of a solute.

Depression of freezing point (ΔTf) is the decrease in the freezing point of a solvent when a non-volatile solute is dissolved in it; ΔTf = Kf × m.
Knowing the mass of solute (w2) and solvent (w1), the molar mass of solute M2 = (Kf × w2 × 1000) / (ΔTf × w1).
By measuring ΔTf experimentally for a known solvent with a known Kf value, the molar mass of the unknown solute can be calculated.

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Question 10

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Define osmosis and osmotic pressure of a solution.

Osmosis is the spontaneous flow of solvent molecules through a semipermeable membrane from a region of lower solute concentration (pure solvent) to a region of higher solute concentration (solution).
Osmotic pressure (Π) of a solution is the excess pressure that must be applied to the solution side to just prevent the flow of solvent through the semipermeable membrane.
For dilute solutions, Π = CRT, where C is the molar concentration, R is the gas constant, and T is the absolute temperature.

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Question 11

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What are colligative properties? List the four colligative properties discussed in the chapter.

Colligative properties are those properties of solutions that depend only on the number of solute particles relative to the total number of particles, irrespective of the chemical identity of the solute.
The four colligative properties are: (1) relative lowering of vapour pressure, (2) elevation of boiling point, and (3) depression of freezing point.
The fourth colligative property is (4) osmotic pressure of the solution.

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Question 12

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What is the van't Hoff factor (i)? What values does it take for association and dissociation?

The van't Hoff factor i is defined as the ratio of normal (theoretical) molar mass to the experimentally determined (abnormal) molar mass, or equivalently, the ratio of observed to calculated colligative property.
For solutes that dissociate in solution (e.g., KCl into K⁺ and Cl⁻), the number of particles increases, so i > 1.
For solutes that associate in solution (e.g., acetic acid dimerising in benzene), the number of particles decreases, so i < 1.

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Question 13

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Distinguish between a saturated solution and an unsaturated solution.

A saturated solution is one in which no more solute can be dissolved at the same temperature and pressure; it is in dynamic equilibrium with undissolved solute and contains the maximum amount of solute.
An unsaturated solution is one in which more solute can still be dissolved at the same temperature; the concentration of solute is less than the maximum possible.
The concentration of solute in a saturated solution is equal to its solubility under the given conditions of temperature and pressure.

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Chapter 1: Solutions | Short Questions

Define a solution and state the role of solvent in determining its properties.

Why is molality preferred over molarity when expressing concentration for experiments involving temperature changes?

State Henry's law for gas solubility and write its mathematical expression.

Explain why the solubility of gases in liquids decreases with an increase in temperature.

Define mole fraction and state the relationship between the mole fractions of all components in a solution.

What are ideal solutions? Give two examples from the chapter.

How does the addition of a non-volatile solute affect the vapour pressure of a solvent? Give the molecular reason.

Define elevation of boiling point and write the expression relating it to molality of the solution.

Define depression of freezing point and state how it can be used to determine the molar mass of a solute.

Define osmosis and osmotic pressure of a solution.

What are colligative properties? List the four colligative properties discussed in the chapter.

What is the van't Hoff factor (i)? What values does it take for association and dissociation?

Distinguish between a saturated solution and an unsaturated solution.

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