Colligative Properties Or how a solute affects a solvent’s properties

What is a colligative property? A physical property of a substance that is dependent upon the number of particles present in it rather than upon the size, mass, and characteristics of those particles Factors include the solvent, the molality of the solution, and the polarity of the solute

Colligative Properties affect… Vapor Pressure Osmotic Pressure Freezing Point Boiling Point

Freezing Point Depression The freezing point of a solvent is lowered when a solute is added to it The new freezing point is determined by considering the solvent, the molality of the solution, and the dissociation of the solute.

Freezing Point Depression The most common solvent is water. A freezing point constant is known for every solvent we will use. The freezing point constant for water is called the k f of water and… k f of water = 1.86°C-kg mol Know this value.

Freezing Point Depression The molality of the solution must be known; remember… m = #mol solute #kg solvent

Freezing Point Depression The dissociation of the solute is represented by the van’t Hoff factor ( i ) –Covalent compounds do not dissociate in solution…C 6 H 12 O 6 does not yield 6 C atoms, 12 H atoms, and 6 O atoms. Rather it remains as one unit of C 6 H 12 O 6 –Thus, the van’t Hoff factor of any covalent compound equals 1. –How do you recognize covalent compounds?

Freezing Point Depression –Ionic compounds do dissociate in solution…NaCl does yield 1 Na + ion and 1 Cl - ion (thus, i = 2) –So, the van’t Hoff factor of any ionic compound is determined by the number of ions yielded. –How do you recognize ionic compounds? –What is the value of i in calcium chloride? What is it in aluminum chloride?

Freezing Point Depression So, piecing it all together, we get the freezing point depression formulas:  T f = (k f )(m)( i ) and then… T fsolution = T fsolvent –  T f

Freezing Point Depression Practice What is the freezing point of a solution made by adding 15.0g of ammonium sulfate to 100g of water? –6.33°C

Boiling Point Elevation The boiling point of a solvent is raised when a solute is added to it The new boiling point is determined by again considering the solvent, the molality of the solution, and the dissociation of the solute.

Boiling Point Elevation A boiling point constant is known for every solvent we will use. The boiling point constant for water is called the k b of water and… k b of water = 0.512°C-kg mol Know this value.

Boiling Point Elevation So, piecing it all together, we get the boiling point elevation formulas:  T b = (k b )(m)( i ) and then… T bsolution = T bsolvent +  T b

Boiling Point Elevation Practice How many kg of sodium chloride would have to be added to 2.00kg of water to raise the boiling point to 110°C? 1.14kg