Introduction Colligative properties are physical properties of a solution that c

Question

Introduction Colligative properties are physical properties of a solution that can change depending on the amount, not identity, of particles dissolved in a solvent. These properties include freezing point depression, boiling point elevation, osmotic pressure and vapor pressure lowering. These properties can be explained by the idea that a solution will contain fewer solvent particles on its surface than the pure solvent, resulting in fewer solvent molecules being able to escape and enter the gas phase. Because colligative properties depend on the number of particles dissolved in solution and not the type, nonelectrolytes and electrolytes must be treated differently. When the one mole of the nonelectrolyte ethylene glycol dissolves in water only one mole of solute particles are generated. However, when one mole of potassium sulfate dissolves in water two moles of K' and one mole of SO ions are formed, resulting in three moles of particles dissolved in water. It would be expected that potassium sulfate should have three times the effect on the colligative properties of water than ethylene glycol. The volume of a solution changes with temperature resulting in a change in the molarity of a solution. To overcome this concentration temperature dependence, the mathematical relationships to describe colligative properties use molality as the concentration. Molality is the moles of solute particles per kilogram mass of solvent. Molality is therefore independent of volume of the solution. In this laboratory you will investigate the effect several different solutes have on the freezing point of aqueous solutions

Explanation / Answer

1.
Molarity is number of moles of solute dissolves per litre of solution.
Molarity,
M = number of moles of solute / volume of solution in L

Molality is number of moles of solute per Kg of solvent.
molality,
m = number of moles of solute / mass of solvent in Kg

Since volume changes with temperature but mass is fixed, molairty changes with temperature but molality doesn't.
So we use molality.

2.
Pure solvent have certain force of attraction holding them together and they account for freezing point. When solute is added to it,they get in between 2 solvent and magnitude of forces holding solvent decreases and hence they tend to melt at lower temperature. This is the reason for depression in freezing point

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