Colligative Properties. Colligative properties Properties that depend on the TOTAL number of...

Colligative Properties

Colligative properties

Properties that depend on the TOTAL number of dissolved particles

Colligative properties

Properties that depend on the TOTAL number of dissolved particles Vapor pressure lowering Boiling point elevation Freezing point depression Osmotic pressure

Vapor pressure lowering

Adding solute leads to more intermolecular attractions

It becomes harder for solvent molecules to escape into the gas phase

Vapor pressure lowering

More solute lower vapor pressure than pure solvent

Raoult’s Law

oAAA PXP

Effects of non-volatile solutes

Boiling Point Elevation

bbp kmT

Freezing Point Depression

ffp kmT

Electrolytes vs. non-electrolytes

Colligative properties depend on total number of dissolved particles

Electrolytes vs. non-electrolytes

Colligative properties depend on total number of dissolved particles Non-electrolytes don’t dissociate Electrolytes DO dissociate into ions in solution

Van’t Hoff factor, i

For non-electrolyte, i = 1 For electolytes, i = # of ions in formula

(theoretical maximum)

Predict the van’t Hoff factor:

Glucose, C6H12O6

NaCl AlCl3 Methanol, CH3OH

Ion pairing

One mole of NaCl does not yield two moles of ions

Some ions will reassociate for a short time

The actual van’t Hoff factor will be slightly lower than predicted

van’t hoff factors

More ion pairing occurs at higher concentrations

van’t hoff factors

More ion pairing occurs at higher concentrations

Incorporating the van’t Hoff factor

Boiling point elevation:

bbp kimT

Incorporating the van’t Hoff factor

Boiling point elevation:

Freezing point depression:

bbp kimT

ffp kimT

Problem 33.5 g of potassium chloride are dissolved in 459 g of water.

Calculate the boiling point and freezing point of the resulting solution.

Problem 17.8 g of an unknown solute are dissolved in 276 g of water. If the

new freezing point is -1.67oC, calculate the molar mass of the unknown substance.