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

Effects of non-volatile solutes

Boiling Point Elevation

Freezing Point Depression

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, C 6 H 12 O 6 NaCl AlCl 3 Methanol, CH 3 OH

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:

Incorporating the van’t Hoff factor Boiling point elevation: Freezing point depression:

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 o C, calculate the molar mass of the unknown substance.