Q. State Raoults law for solutions of volatile liquids. Taking suitable examples explain the meaning of positive and negative deviations from Raoults law.
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Raoults Law states that :For a solution of volatile liquids, the partial vapour pressure of each components directly proportional to its mole fraction.
pi= xi * pi^o
Positive deviation from Raoults law When vapour pressure of solution is higher the intermolecular forces of attraction between solute-solvent molecules are weaker than those between solute-solute molecules and solvent-solvent molecules. Therefore, solvent molecules can easily escape, resulting in increase in vapour pressure.
Example: Ethanol and acetone mixture.
Negative deviation from Raoults law When vapour pressure of solution is lower the intermolecular forces of attraction between solute-solute molecules and solvent-solvent molecules are weaker than those between solute-solvent molecules. This results in the decreasing of vapour pressure.
Example: Chloroform and acetone mixture.
pi= xi * pi^o
Positive deviation from Raoults law When vapour pressure of solution is higher the intermolecular forces of attraction between solute-solvent molecules are weaker than those between solute-solute molecules and solvent-solvent molecules. Therefore, solvent molecules can easily escape, resulting in increase in vapour pressure.
Example: Ethanol and acetone mixture.
Negative deviation from Raoults law When vapour pressure of solution is lower the intermolecular forces of attraction between solute-solute molecules and solvent-solvent molecules are weaker than those between solute-solvent molecules. This results in the decreasing of vapour pressure.
Example: Chloroform and acetone mixture.
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