Saturday 12 August 2017

CBSE Class 12 - Chemistry - Chapter 2 - Solutions (Important Points) (#cbseNotes)

Solutions - Important Points

Class 12 - Chemistry - Chapter 2
CBSE Class 12 - Chemistry - Chapter 2 - Solutions (Important Points) (#cbseNotes)

1. A solution is a homogeneous mixture of two or more substances.

2. Solutions are classified as solid, liquid and gaseous solutions.

3. The component that is having more number of moles is known as solvent.

4. Solvent determines the physical state of the solution.

5. Water is an universal solvent.

6. The concentration of a solution is expressed in terms of
- mole fraction,
- molarity,
- molality and
- in percentages.

7. Mole fraction (X) is a unitless quantity.

8. Molality (m) and mole fraction are temperature independent quantities whereas molarity decreases with increase in temperature.

9. The dissolution of a gas in a liquid is governed by Henry’s law, according to which, at a given temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas.

10. As the temperature increases Henry’s law constant, KH increases so the lower is the solubility of the gas in the liquid.

11. 11.7% w/w Helium is added to air used by scuba divers due to its low solubility in the blood.

12.  The vapour pressure of the solvent is lowered by the presence of a non-volatile solute in the solution and this lowering of vapour pressure of the solvent is governed by Raoult’s law.

13. According to Raoult’s law, the relative lowering of vapour pressure of the solvent over a solution is equal to the mole fraction of a non-volatile solute present in the solution.

14. However, in a binary liquid solution, if both the components of the solution are volatile then another form of Raoult’s law is used. Mathematically, this form of the Raoult’s law is stated as:

p(total) = p⁰₁x₁ + p⁰₂x₂

15. Raoult’s law becomes a special case of Henry’s law in which KH becomes equal to PA⁰ , i.e., vapour pressure of pure solvent.

16. Solutions which obey Raoult’s law over the entire range of concentration are called ideal solutions.

17. Two types of deviations from Raoult’s law, called positive and negative deviations are observed. Azeotropes arise due to very large deviations from Raoult’s law.

18. Azeotropes having the same composition in liquid and vapour phase and boil at a constant temperature and therefore can’t be distilled.

19. Maximum boiling azeotropes form when solutions exhibit negative deviation from Raoult’s law whereas minimum boiling azeotropes form when solutions exhibit positive deviation from Raoult’s law.

20.  The properties of solutions which depend on the number of solute particles and are independent of their chemical identity are called colligative properties.

21. These are lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure. The process of osmosis can be reversed if a pressure higher than the osmotic pressure is applied to the solution.

22. Colligative properties have been used to determine the molar mass of solutes. Solutes which dissociate in solution exhibit molar mass lower than the actual molar mass and those which
associate show higher molar mass than their actual values.

23. Quantitatively, the extent to which a solute is dissociated or associated can be expressed by Van’t Hoff factor i. Van’t Hoff factor (i) is the ratio of the observed value of the colligative property in solution to the theoretically calculated value of the colligative property.

(a) A non-volatile solute undergoes dissociation, then i > 1.

(b) A non-volatile solute undergoes association, then i < 1.

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