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Molality is a measure of concentration

Molality = moles of solute / mass of solvent (kg)

What are colligative properties"

Colligative properties of solutions are properties that rely upon the number of dissolved particles in a solvent, but not upon the type of solutes.

Colligative properties include freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure.

For example, if you add salt to water, you can lower the freezing point of water so that it freezers below 0ºC. The amount of temperature decreased is known as freezing point depression.

Salt and sugar solution

Comparing the colligative properties of a salt and sugar solution
Example:

A sugar solution has a concentration of 1m. A salt solution has a concentration of 0.5m. What is the effect on the colligative properties of both solutions?

Answer:

Using boiling point elevation as an example.

ΔT = i x Kb x m

where ΔT = boiling point elevation

i = Van't Hoff factor

m = molality of solution

Boiling point of sugar solution

ΔT = i x Kb x m

i = 1 : using Van't Hoff factor of 1 because glucose does not dissociate in water

Kb : the boiling point constant for water is 0.512 ºc kg/mol

= 1 x 0.512 ºc kg/mol x 1 mol/kg

= 0.51ºC

Boiling point of salt solution

i = 2 : using Van't Hoff factor of 2 because sodium chloride dissociates into 2 particles in water.

Kb : the boiling point constant for water is 0.512 ºc kg/mol

= 2 x 0.512 ºc kg/mol x 0.5 mol/kg

= 0.51ºC

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