1. -- these depend on the concentration of particles in a solution, but not... the kind of particles (A) Adding a volatile/nonvolatile solute to a solvent increases/ decreases the solution’s vapor pressure (VP). Raoult’s law: P A, mix = VP of particle type “A” above the mixture X A = mole fraction of particle type “A” in the mixture P o A = VP of “pure A” P A, mix = X A P o A Colligative Properties Francois Marie Raoult (1830 – 1901) Use this eq. once for every substance in mix. that has a measureable VP.

2. Ideal solutions obey Raoult’s law. Such solutions have... -- a low concentration -- solute and solvent particles that are similar in size and have similar IMFs (i.e., are relatively dilute)

3. A solution of sodium chloride and water has a vapor pressure of 0.854 atm at 100. o C. Find the mole fraction of sodium chloride. P A, mix = X A P o A (Key #1: NaCl is nonvolatile, so VP above the mixture is due entirely to H 2 O.) 0.854 atm = X H2O (1.00 atm)  P H2O, mix = X H2O P o H2O  X H2O = 0.854 and so X is 0.146 for all other particles because then… X Na+ + X Cl– = 0.146 (Key #2: The VP of H 2 O @ 100. o C should be recognizable.) But here… X NaCl = 0.073 If the solute were sucrose, X sucrose = 0.146.

4. A solution contains 89.7 g ethanol and 241.4 g water. What is the vapor pressure above the mixture at 100. o C, if ethanol’s vapor pressure at this temp. is 1694 torr? (Find VP of each substance, then use Dalton’s law to find P tot.) P tot, mix = P H2O, mix + P eth, mix = 1.95 mol eth = 13.41 mol H 2 O 89.7 g eth 241.4 g H 2 O CH 3 CH 2 OH X H2O = 0.873 X eth = 0.127 P H2O, mix = X H2O P o H2O P eth, mix = X eth P o eth = 0.127(1694) = 0.873(760) = 215 torr = 664 torr P tot = 879 torr