Colligative Properties Colligative properties l Properties that depend

• Slides: 19

Colligative Properties

Colligative properties l Properties that depend on the TOTAL number of dissolved particles

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

Vapor pressure lowering l l Adding solute leads to more intermolecular attractions It becomes harder for solvent molecules to escape into the gas phase

Vapor pressure lowering l More solute lower vapor pressure than pure solvent l Raoult’s Law

Effects of non-volatile solutes

Boiling Point Elevation

Freezing Point Depression

Electrolytes vs. nonelectrolytes l Colligative properties depend on total number of dissolved particles

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

Van’t Hoff factor, i l l For non-electrolyte, i = 1 For electolytes, i = # of ions in formula (theoretical maximum)

Predict the van’t Hoff factor: l l Glucose, C 6 H 12 O 6 Na. Cl Al. Cl 3 Methanol, CH 3 OH

Ion pairing l l l One mole of Na. Cl 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 l More ion pairing occurs at higher concentrations

van’t hoff factors l More ion pairing occurs at higher concentrations

Incorporating the van’t Hoff factor l Boiling point elevation:

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

Problem l 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 l 17. 8 g of an unknown solute are dissolved in 276 g of water. If the new freezing point is -1. 67 o. C, calculate the molar mass of the unknown substance.