Solutions Colligative Properties • vapor pressure lowering Raoult’s Law: PA = XAP°A • boiling point elevation ΔTb = Kbm • freezing point depression ΔTf = Kfm • osmotic pressure € π=nV⎛ ⎝ ⎜ ⎞ ⎠ ⎟ RT = M RTColloids 1 Factors that affect solubility • intermolecular interactions (like dissolves like) • temperature • pressure Mary J. Bojan Chem 110SOLUBILITY FACTORS THAT AFFECT SOLUBILITY Temperature – Effect of Temperature on solubility of ionic solids – Effect of temperature on solubility of gases Type of intermolecular interactions and their strengths (LIKE DISSOLVES LIKE) Pressure 2 Mary J. Bojan Chem 110How does solubility of ionic solids change with temperature? 3 Mary J. Bojan Chem 110How does solubility of gases change with temperature? 4 Mary J. Bojan Chem 110In general: like dissolves like – polar solvents dissolve polar (and ionic solutes) – nonpolar solvents dissolve non-polar solutes If intermolecular forces broken and formed are ~ the same, dissolution is probable. How do intermolecular forces affect solubility? Which one of the following will be most soluble in benzene (C6H6(l))? 1. H2O(l) 2. CH3OH(l) 3. HCl(l) 4. CH3CH2OH(l) 5. heptane 5 Mary J. Bojan Chem 110Like dissolves like Alcohol solubility in water at 25°C (g/100g of H2O) CH3OH total CH3CH2OH total CH3CH2CH2OH total CH3CH2CH2CH2OH 8.06 CH3CH2CH2CH2CH2OH 2.82 CH3CH2CH2CH2CH2CH2OH 0.62 6 As length of hydrocarbon chain increases, solubility of alcohol in water decreases. Mary J. Bojan Chem 110Put these in order of increasing solubility in water 7 Mary J. Bojan Chem 110Which vitamin is soluble in fat? Which vitamin is water soluble? 8 Fat soluble vitamins can be stored in the body Water soluble vitamins are not stored in appreciable amounts Mary J. Bojan Chem 110Increase Psolute(gas); increase solubility Decrease Psolute(gas); decrease solubility How does solubility of gases change with pressure? amount of gas dissolved (Cg) is proportional to the partial pressure of that gas (Pg) above the solution. k = Henry’s Law constant 9 HENRY’S Law Cg = k Pg solubility Cg partial pressure Pg Mary J. Bojan Chem 110Deep Sea Diving Solubility increases with increasing P − amount of N2 in blood increases as depth increases. As divers rise, N2 bubbles out of blood causing painful “bends”. Solution: use He/O2 mixture when diving deep. He is not as soluble in blood. 10 Mary J. Bojan Chem 110Ammonia Fountain H2O + phenolphthalein (pink in base) 11 Mary J. Bojan Chem 110Test yourself To increase the solubility of N2 in water 1. increase T 2. decrease T 3. increase P 4. decrease P How will the solubility of KClO3 be effected by an increase in pressure? 1. increase solubility 2. decrease solubility 3. no effect on solubility 12 Mary J. Bojan Chem 110COLLIGATIVE PROPERTIES Properties that depend on the number of solute particles (collections) in solution but not on the identity of the solute. 13 Mary J. Bojan Chem 110 Examples of Colligative properties • vapor pressure lowering • boiling point elevation • freezing point depression • osmotic pressureExample: What is the total particle concentration in a 0.2M solution of Pb(NO3)2 ? (salt, strong electrolyte) [Pb+2]= [NO3−]= total concentration of particles = COLLIGATIVE PROPERTIES 14 Mary J. Bojan Chem 110 What is the total particle concentration 0.2M solution of CH3COOH (acetic acid)? (weak acid, weak electrolyte) CH3COOH(aq) H+(aq) + CH3COO−(aq)VAPOR PRESSURE LOWERING Raoult’s Law: PA = XAP°A PA ≡ vapor pressure of solution XA ≡ mole fraction of solvent P°A ≡ vapor pressure of pure solvent 15 Mary J. Bojan Chem 110 Vapor pressure lowering is a colligative property It depends on the concentration but not on the nature of the solute. rate of evaporation ↓ vapor pressure of SOLVENT ↓VAPOR PRESSURE LOWERING 16 Mary J. Bojan Chem 110BOILING POINT ELEVATION FREEZING POINT DEPRESSION ΔTb = Kbm Kb = molal boiling point elevation constant ΔTf = Kfm Kf = molal freezing point depression constant m ≡ molality of the solution 17 Mary J. Bojan Chem 110 Kb and Kf are tabulated for different solvents Eg. water Kb= 0.52°C/m Kf= 1.86°C/m benzene Kb= 2.53°C/m Kf= 5.12°C/mExample What is the freezing point of seawater assuming all of the salinity* is due to NaCl? (MW of NaCl = 58.5 g/mol) *ocean salinity is ca. (35 g salt)/(1 kg seawater) mainly: Cl–, Na+, SO42-, Mg2+, Ca2+, K+ 18 Mary J. Bojan Chem 110Osmosis flow of molecules through a membrane Osmotic pressure: π Pressure needed to stop the flow of molecules through a membrane € π = nV⎛ ⎝ ⎜ ⎞ ⎠ ⎟ RT = MRTunits of atm molarity 19 Mary J. Bojan Chem 110Osmosis through a membrane Hypertonic: solution: water moves out of cell Hypotonic solution: water moves into cell; can cause cell to rupture. Isotonic: solution that has same osmotic pressure as cell. 20 Mary J. Bojan Chem 110Reverse Osmosis exert pressure on a concentrated solution, forcing solvent molecules through a membrane. Solute molecules are trapped in concentrated portion.)Used to purify water. 21 Mary J. Bojan Chem 110An aqueous solution contains 3.50 mg of protein dissolved in sufficient water to form 5.00 mL of solution. The osmotic pressure of the solution at 25°C was found to be 1.54 torr. Calculate the molar mass of the protein. 22 Mary J. Bojan Chem 110COLLOIDS true colloidal mixture solution dispersion uniform uniform non-uniform molecules 20-2000Å sedimentary particles particles NaCl milk (fat particles) silt H2O fog (water droplets) small large Particle size 23 Mary J. Bojan Chem 110Light scattering Tyndall effect: light is scattered when λ of light ≅ particle size For colloids (size~ 200nm) λ is in visible region: Light passes through colloidal suspensions with scattering (milk, fog). 24 Mary J. Bojan Chem 110 For molecules (~5Å) λ in X-ray region • visible light passes through solutions without scattering • Atmosphere scatters small λ light Explains why the sky is blueTest Question Which of the following aqueous solutions has the greatest total concentration of ions? 1. 0.2M NH4NO3 2. 0.2M Pb(NO3)2 3. 0.2M Na2SO4 4. 0.2M AlPO4 5. 0.2M AlBr3 6. 0.2M CH3COOH (acetic
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