Colligative Properties of Solutions (chapter 16) Colligative properties = physical properties of solutions that depend on the # of particles dissolved, not the kind of particle.

Colligative Properties Lowering vapor pressure Raising boiling point Lowering freezing point Generating an osmotic pressure

2 to focus on… Lowering vapor pressure Raising boiling point Lowering freezing point Generating an osmotic pressure

Boiling Point Elevation a solution that contains a nonvolatile solute has a higher boiling pt than the pure solvent; the boiling pt elevation is proportional to the # of moles of solute dissolved in a given mass of solvent. Like when adding salt to a pot of boiling water to make pasta 

Boiling Point Elevation Tb = kbm where: Tb = elevation of boiling pt m = molality of solute (mol solute/kg solvent) kb = the molal boiling pt elevation constant kb values are constants; see table 16.3 pg. 495 kb for water = 0.52 °C/m

Ex: What is the normal boiling pt of a 2 Ex: What is the normal boiling pt of a 2.50 m glucose, C6H12O6, solution? “normal” implies 1 atm of pressure Tb = kbm Tb = (0.52 C/m)(2.50 m) Tb = 1.3 C Tb = 100.0 C + 1.3 C = 101.3 C

Freezing/Melting Point Depression The freezing point of a solution is always lower than that of the pure solvent. Like when salting roads in snowy places so the roads don’t ice over or when making ice cream 

Freezing/Melting Point Depression Tf = kfm where: Tf = lowering of freezing point m = molality of solute kf = the freezing pt depression constant kf for water = 1.86 °C/m kf values are constants; see table 16.2 pg. 494

Ex: Calculate the freezing pt of a 2.50 m glucose solution. Tf = kfm Tf = (1.86 C/m)(2.50 m) Tf = 4.65 C Tf = 0.00C - 4.65 C = -4.65C

Electrolytes and Colligative Properties • Colligative properties depend on the # of particles present in solution. • Because ionic solutes dissociate into ions, they have a greater effect on freezing pt and boiling pt than molecular solids of the same molal conc.

Electrolytes and Colligative Properties For example, the freezing pt of water is lowered by 1.86°C with the addition of any molecular solute at a concentration of 1 m. Such as C6H12O6, or any other covalent compound However, a 1 m NaCl solution contains 2 molal conc. of IONS. Thus, the freezing pt depression for NaCl is 3.72°C…double that of a molecular solute. NaCl  Na+ + Cl- (2 particles)

Electrolytes - Boiling Point Elevation and Freezing Point Depression The relationships are given by the following equations: Tf = kf ·m·n or Tb = kb·m·n Tf/b = f.p. depression/elevation of b.p. m = molality of solute kf/b = b.p. elevation/f.p depression constant n = # particles formed from the dissociation of each formula unit of the solute

Ex: What is the freezing pt of a 1.15 m sodium chloride solution? NaCl  Na+ + Cl- n=2 Tf = kf·m·n Tf = (1.86 C/m)(1.15 m)(2) Tf = 4.28 C Tf = 0.00C - 4.28 C = -4.28C