Solutions - Class 12 Jharkhand Board Chemistry

Solutions and Concentration Terms

This topic covers the key concepts, definitions, and formulas related to solutions and their concentration terms, including mass percentage, volume percentage, mole fraction, solubility, Henry's law, Raoult's law, and related phenomena. The focus is on the calculation methods, conceptual understanding, and exam-relevant distinctions for each term.

Mass Percentage, Volume Percentage, and Mass by Volume Percentage

Mass percentage expresses the amount of solute in a solution as a percentage of the total mass. Volume percentage is used when both solute and solvent are liquids, and mass by volume percentage is used when solute is solid…

Solutions and Concentration Terms

This topic covers the key concepts, definitions, and formulas related to solutions and their concentration terms, including mass percentage, volume percentage, mole fraction, solubility, Henry's law, Raoult's law, and related phenomena. The focus is on the calculation methods, conceptual understanding, and exam-relevant distinctions for each term.

Mass Percentage, Volume Percentage, and Mass by Volume Percentage

Mass percentage expresses the amount of solute in a solution as a percentage of the total mass. Volume percentage is used when both solute and solvent are liquids, and mass by volume percentage is used when solute is solid and solvent is liquid.

• Mass Percentage:
  • Formula: $\text{Mass \%} = \frac{\text{Mass of solute}}{\text{Total mass of solution}} \times 100$
  • Example: If 20 g solute is dissolved in 80 g solvent, total mass = 100 g, so mass % = $20/100 \times 100 = 20\%$.
• Volume Percentage:
  • Formula: $\text{Volume \%} = \frac{\text{Volume of solute}}{\text{Total volume of solution}} \times 100$
  • Note: Total volume is not always the sum of individual volumes due to lack of conservation of volume.
• Mass by Volume Percentage:
  • Defined as mass of solute dissolved in 100 ml of solution.

Parts Per Million (ppm)

Parts per million is used to express very small concentrations, especially when solute is present in trace amounts.

• Definition: When solute is present in very small quantities, ppm is used to avoid writing many zeros.
• Formula: $\text{ppm} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 10^6$
• Used for concentrations like dissolved oxygen in water or trace metals in biological samples.

Mole Fraction

Mole fraction is a unitless concentration term representing the ratio of moles of a component to the total moles in the solution.

• Formula: For component A, $X_A = \frac{\text{Moles of A}}{\text{Total moles in solution}}$
• For a binary solution with components A and B: $X_A + X_B = 1$
• Properties:
  • Mole fraction is unitless.
  • It is temperature independent.
  • The sum of mole fractions of all components in a solution is always unity.

Solubility and Factors Affecting Solubility

Solubility is the maximum amount of a substance that can dissolve in a fixed amount of solvent at a fixed temperature.

• Definition: The maximum amount of a substance that can be dissolved in a fixed amount of solvent at a fixed temperature.
• Measurement: Solubility can be measured using mole fraction.
• Factors Affecting Solubility:
  • Nature of solute and solvent (like dissolves like: polar solutes dissolve in polar solvents, non-polar in non-polar).
  • Temperature: For endothermic dissolution, solubility increases with temperature; for exothermic, it decreases.
  • Pressure: Affects solubility of gases in liquids, not solids or liquids in liquids.

Saturated and Unsaturated Solutions

A saturated solution cannot dissolve any more solute at a given temperature, while an unsaturated solution can.

• Saturated Solution: No more solute can be added at a given temperature.
• Unsaturated Solution: More solute can still be dissolved.
• Key Point: The concept is analogous to a filled capacity—once full, no more can be added.

Dissolution and Crystallization

Dissolution is the process of solute dissolving in solvent, while crystallization is the reverse process where solute separates out as a solid.

• Dissolution: Solute breaks into smaller particles and disperses in solvent.
• Crystallization: Dissolved solute particles aggregate and separate out as solid crystals.
• Both processes are reversible and depend on solution conditions.

Effect of Temperature and Pressure on Solubility

Temperature and pressure have different effects on solubility depending on the nature of solute and solvent.

• Temperature:
  • For endothermic dissolution: Solubility increases with temperature.
  • For exothermic dissolution: Solubility decreases with temperature.
• Pressure:
  • No significant effect on solubility of solids and liquids in liquids (due to incompressibility).
  • Significant effect on gases in liquids; solubility increases with pressure.

Solubility of Gases in Liquids and Henry’s Law

The solubility of gases in liquids is governed by Henry’s law, which relates solubility to the partial pressure of the gas above the solution.

• Henry’s Law Statement: The solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.
• Formula: $P = K_H \times X$ (where $P$ is partial pressure, $K_H$ is Henry’s constant, $X$ is mole fraction of gas)
• Key Points:
  • Different gases have different $K_H$ values.
  • Higher $K_H$ means lower solubility.
  • $K_H$ depends on the nature of the gas.

Applications of Henry’s Law

Henry’s law explains several real-life phenomena and has important applications.

• Soft Drinks: CO$_2$ is dissolved in drinks under high pressure to increase solubility; opening the bottle reduces pressure, releasing gas.
• Bends (Decompression Sickness): Divers experience increased gas solubility at depth due to high pressure; rapid ascent decreases pressure, causing dissolved gases to form bubbles in blood, leading to pain and harm.
• Anoxia: At high altitudes, lower atmospheric pressure reduces oxygen solubility in blood, leading to insufficient oxygen supply to the brain.

Vapour Pressure, Volatile and Non-Volatile Substances

Vapour pressure is the pressure exerted by vapour in equilibrium with its liquid in a closed container at a given temperature.

• Volatile Substance: Forms vapour easily (e.g., water).
• Non-Volatile Substance: Does not form vapour (e.g., stone).
• Vapour Pressure Definition: The pressure exerted by vapour when in equilibrium with its liquid in a closed container at a given temperature.
• Equilibrium Condition: Rate of condensation equals rate of evaporation.

Raoult’s Law for Volatile Liquid-Liquid Solutions

Raoult’s law describes the partial vapour pressure of each component in an ideal solution.

• Statement: For volatile liquid-liquid solutions, the partial vapour pressure of each component is directly proportional to its mole fraction in the solution.
• Formulas:
  • $P_A = P_A^0 \times X_A$
  • $P_B = P_B^0 \times X_B$
  • $P_{\text{total}} = P_A + P_B = P_A^0 X_A + P_B^0 X_B$
  • Since $X_A + X_B = 1$, $P_{\text{total}}$ can be rearranged as $P_A^0 (1 - X_B) + P_B^0 X_B$
• Partial Vapour Pressure: The contribution of each component to the total vapour pressure in a mixture.

Factors Affecting Vapour Pressure

Vapour pressure depends on temperature and the nature of the liquid, but not on the amount, surface area, or shape of the container.

• Key Points:
  • Vapour pressure increases with temperature.
  • More volatile liquids have lower boiling points.
  • Vapour pressure is independent of the amount of substance, surface area, and shape of the container.

Exam-Oriented Objective Points and Sample Calculations

Several objective questions and calculation methods are directly relevant for exams.

• Mole Fraction: Unitless and temperature independent.
• Molality: Number of moles of solute per kg of solvent; temperature independent.
• Sample Calculation: For 2 g NaOH in 200 ml solution:
  • Molar mass of NaOH = 40 g/mol.
  • Moles of NaOH = $2/40 = 0.05$ mol.
  • Volume in liters = $200/1000 = 0.2$ L.
  • Molarity = $0.05/0.2 = 0.25$ M.
• Objective Question: Molality is not affected by temperature.

These points and formulas are frequently tested in exams and should be memorized for quick recall.