2 Gases dissolving in liquids Pressure and temperature influence gas solubilitySolubility directly proportional to gas pressureHenry’s Law:Sg = kHPgSg = gas solubility (M = mol/L)kH = Henry’s law constant (unique to each gas; M/mm Hg)Pg = partial pressure of gaseous solute (mm Hg)
4 Example27.0 g of acetylene gas dissolves in 1.00 L of acetone at 1.00 atm partial pressure of acetylene.If the partial pressure of acetylene is increased to 6.00 atm, what is the solubility of acetylene in acetone in mol/L? MW of acetylene = g/mol27.0 g x (mol/ g) x (1/1.00 L) = 1.04 MSg = kHPg1.04 M = kH x 1.00 atmkH = 1.04 M/atmSg = (1.04 M/atm) x 6.00 atm= 6.24 MCould also solve this by:(Sg1/Pg1) = (Sg2/Pg2)How did I come up with this?
5 ProblemThe partial pressure of oxygen gas, O2, in air at sea level is 0.21 atm.Using Henry’s Law, calculate the molar concentration of oxygen gas in the surface water (at 20°C) of a lake saturated with air given that the solubility of O2 at 20°C and 1.0 atm pressure is 1.38•10-3 M.
7 They call it “pop” in the Midwest Drinks carbonated under high pressureAbove 90 atmUnder CO2 atmosphereOnce bottle opened, partial pressure of gas above soda plummetsCO2 solubility decreases drasticallyGas bubbles out of solnOnce the fizz is gone, it can never be regainedTruly, one of the existential tragedies of this universe
8 The bends Deeper diving has higher pressures Must use breathing tankIf it contains N2 then higher pressure forces N2 to dissolve in higher amounts in bloodIf ascension too fast, lower pressure causes N2 to start bubbling out of blood too quicklyRupturing of arteriesExcruciatingly painful deathMust be rushed to hyperbaric chamberTanks now don’t use N2, but HeWhy?
9 Effects of temp on solubility Obviously, as temp increases, solubility decreasesSince increasing heat causes gases to dissolve out (endothermic) dissolving gases is an exothermic process
10 Another look at gas solubility: Le Châtelier’s Principle Explains temperature relevance of solubilityFor systems in equilibrium, change in one side causes system to counteract on other side:Gas + liquid solvent sat. soln + heatSo add heat, rxn goes to left by kicking out gasAdd gas, rxn goes to right by saturating soln & giving off heat
11 Solubility of solids based on temperature In general, solubility increases w/ increasing tempBut exceptionsNo general behavior pattern noted
12 CrystalsOne can separate impure dissolved salts by reducing temperatureImpurity or desired product crystallizes out at specific temp as solubility collapses
13 Colligative properties Vapor and osmotic pressures, bp, and mp are colligative propertiesDepend on relative # of solute and solvent particles
14 Vapor Pressure Remember: Equilibrium vapor pressurePressure of vapor when liq and vapor in equilibrium at specific tempVapor pressure of soln lower than pure solvent vapor pressureVapor pressure of solvent relative # of solvent molecules in solni.e., solvent vapor pressure solvent mole fraction
15 Raoult’s Law Psolution = Xsolvent P°solvent So if 75% of molecules in soln are solvent molecules (0.75 = Xsolvent)Vapor pressure of solvent (Psolvent) = 75% of P°solvent
16 ProblemThe vapor pressure of pure acetone (CH3COCH3) at 30°C is atm. Suppose 15.0 g of benzophenone, C13H10O (MW = g/mol), is dissolved in 50.0 g of acetone (MW = g/mol).Calculate the vapor pressure of acetone above the resulting solution.
18 ProblemThe vapor pressure of pure liquid CS2 is atm at 20°C. When 40.0 g of rhombic sulfur (a naturally occurring form of sulfur) is dissolved in 1.00 kg of CS2, the vapor pressure falls to atm.Determine the molecular formula of rhombic sulfur.
20 Limitations of Raoult’s Law Doesn’t take into consideration attractive forces in solnsFor ideal soln (to right), forces between solute/solvent molecules = forces w/in pure solventThus, Ptot = PA + PBLike graph to rightFine for similarly constructed molecules (hydrocarbons)London dispersion forces are weakest
21 Solute-solvent > solv-solv Decreases vapor pressuredecreased volatilityGet lower vapor pressure than calculatedEx:CHCl3 & C2H5OC2H5H on former H-bonds to latterDoes it increase or decrease the latter’s IMF?
22 Solute-solvent < solv-solv Increases vapor pressureincreased volatilityGet higher vapor pressure than calculatedEx:C2H5OH and H2OFormer disrupts H-bonding of latterDoes it increase or decrease the latter’s IMF?
23 Nonvolatile solute added to solvent SaltsLower vapor pressure of solventMake solvent less volatile
24 Nonvolatile solute added to solvent Raises bpLowers mpWhy?Adding more nonvolatile solute or increasing solute molalitydecreases vapor pressure even morePhase diagram to rightPure water (black)Adulterated water (pink)
25 Bp and molality relationship Tbp = Kbp msoluteKbp = molal boiling pt elevation constant for solvent (°C/m)Bp elevation, Tbp, directly proportional to solute molality
27 Antifreeze Propylene glycol 1,2-propanediol Formerly used ethylene glycolPhased outPoisonousLowers melting ptIncreases boiling ptReduces risk of radiator “boiling over”Appreciated during the summer months in the desert
28 Example Pure toluene (C7H8) has a normal boiling point of 110.60°C. A solution of 7.80 g of anthracene (C14H10) in g of toluene has a boiling point of °C.Calculate Kb for toluene.Tbp = Kbp msoluteTbp = °C °C = 1.46°C7.80g x (mol/178.23g) = 4.38 x 10-2 mol(4.38 x 10-2 mol/ kg) = m1.46°C/0.438 m = 3.33°C/m
30 ProblemBarium chloride has a freezing point of 962°C and a Kf of 108 °C/m.A solution of 12.0 g of an unknown substance dissolved in 562 g of barium chloride gives a freezing point of 937°C.Determine the molecular weight of the unknown substance.
32 Solutions containing ions: their colligative properties Colligative properties based on amount of solute/solventMolality of ions depend on number of constituents in cmpdDifferent for ionic vs. covalent cmpdsEx:NaCl ionizes into two ionsSo 0.5 m NaCl has 0.5 x 2 m = 1 mtotBenzene doesn’t ionizeSo 0.5 m benzene = 0.5 mtotUsing equation w/out above factor will lead to values that are off
33 How to correct for it: the van’t Hoff factor i = the number of solute particles after dissolvingColligative properties are larger for electrolytes than for nonelectrolytes of the same molalityWhy? (Hint: solve the below)Give the i-values for: methanol, CaSO4, BaCl2Tfp (measured) = Kfp m i
34 ProblemHow many grams of Al(NO3)3 must be added to 1.00 kg of water to raise the boiling point to 105.0°CKb = 0.51 °C/mMW = g/mol
38 More…Pressure of column of soln = pressure of water moving through membraneOsmotic pressure = pressure made by column of soln = diff of heights = cRTc = mol/L = MR = L atm/(mol K) ideal gas lawT = in Kelvin = atmUseful for measuring MM of biochemical macromoleculesProteins and carbs