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Science Class 12 - Solutions Notes

Comprehensive study notes for Class 12 - Solutions olympiad preparation

Solutions

Welcome to the chapter on Solutions for Class 12. In this chapter, you will learn about the types of solutions, concentration terms, solubility, colligative properties, and their applications. By the end of this chapter, you will be able to solve numerical problems and understand the importance of solutions in daily life and industry.

Key Concepts

  • Solution: A homogeneous mixture of two or more substances.
  • Solute: The substance that is dissolved.
  • Solvent: The substance in which the solute is dissolved.
  • Concentration: The amount of solute present in a given quantity of solvent or solution.

Types of Solutions

  • Solid in liquid: Sugar in water
  • Gas in liquid: Oxygen in water
  • Liquid in liquid: Alcohol in water
  • Gas in gas: Air (mixture of gases)

Concentration Terms

  • Mass percentage (w/w): (Mass of solute / Mass of solution) × 100
  • Volume percentage (v/v): (Volume of solute / Volume of solution) × 100
  • Mole fraction (X): Moles of component / Total moles of all components
  • Molarity (M): Moles of solute / Volume of solution in litres
  • Molality (m): Moles of solute / Mass of solvent in kg

Solubility

Solubility is the maximum amount of solute that can be dissolved in a given amount of solvent at a specific temperature.

  • Solubility of solids in liquids usually increases with temperature.
  • Solubility of gases in liquids decreases with increase in temperature and increases with pressure.

Vapour Pressure of Solutions

  • Raoult’s Law: The partial vapour pressure of each component is proportional to its mole fraction in the solution.
  • For ideal solutions: Ptotal = PA0XA + PB0XB

Colligative Properties

Colligative properties depend on the number of solute particles, not their nature.

  • Relative lowering of vapour pressure
  • Elevation of boiling point
  • Depression of freezing point
  • Osmotic pressure

Abnormal Molar Masses and van’t Hoff Factor

  • van’t Hoff factor (i): Accounts for association or dissociation of solute particles.
  • i = observed colligative property / calculated colligative property

Applications

  • Salting of roads in winter (depression of freezing point)
  • Antifreeze in car radiators
  • Preservation of food using salt or sugar
  • IV fluids in hospitals (osmotic pressure)

Practice Questions

  1. Define molarity and molality. How are they different?
  2. State Raoult’s Law for a solution of two volatile liquids.
  3. Calculate the mole fraction of NaCl in a solution containing 5.85 g NaCl (molar mass = 58.5 g/mol) dissolved in 100 g of water.
  4. What is the effect of temperature on the solubility of gases in liquids?
  5. Explain the term ‘colligative properties’ with examples.

Challenge Yourself

  • A solution is prepared by dissolving 10 g of urea (molar mass = 60 g/mol) in 250 g of water. Calculate the molality of the solution.
  • Why is the freezing point of sea water lower than that of pure water?
  • How does the van’t Hoff factor affect the calculation of molar mass from colligative properties?

Did You Know?

  • The Dead Sea is so salty that people can easily float on its surface!
  • Osmosis is essential for the survival of all living cells.

Glossary

  • Solution: A homogeneous mixture of two or more substances.
  • Solute: The substance that is dissolved in a solution.
  • Solvent: The substance in which the solute is dissolved.
  • Colligative Property: A property that depends on the number of solute particles.
  • Osmosis: The movement of solvent through a semipermeable membrane from a region of lower solute concentration to higher solute concentration.

Answers to Practice Questions

  1. Molarity (M): Moles of solute per litre of solution.
    Molality (m): Moles of solute per kg of solvent.
    Difference: Molarity depends on volume of solution, molality depends on mass of solvent.
  2. The partial vapour pressure of each component is proportional to its mole fraction in the solution.
  3. Moles of NaCl = 5.85 / 58.5 = 0.1 mol; Moles of water = 100 / 18 = 5.56 mol; Mole fraction of NaCl = 0.1 / (0.1 + 5.56) ≈ 0.018
  4. Solubility of gases in liquids decreases with increase in temperature.
  5. Colligative properties depend on the number of solute particles, e.g., boiling point elevation, freezing point depression, osmotic pressure.

Understanding solutions is key to chemistry and many real-life applications!

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