COLLIGATIVE PROPERTIES

Transcription

1 COLLIGATIVE PROPERTIES Depend on the number of solute particles in solution but not on the identity of the solute Vapor pressure lowering Boiling point elevation Freezing point depression Osmotic pressure EXAMPLES 0.5 m solution of Pb(NO 3 ) m Pb 2+ and 1.0 m NO m total ions 0.5 m HC 2 H 3 O 2 (acetic acid) HC 2 H 3 O 2 H + + C 2 H 3 O 2 solution is between 0 and 1.0 m in total ions solution is between 0.5 and 1.0 m in all species

2 EXPRESSING CONCENTRATION weight percent = mass component total mass x 100 (%) mole fraction of component = moles component total moles (fraction) molarity = molality = moles solute liters solution moles solute mass solvent (kg) (M) (m)

3 ELECTROLYTES A substance that yields ions when dissolved in water is an electrolyte Strong electrolytes completely ionized in solution good conductors Weak electrolytes partially ionized in solution poor conductors Nonelectrolytes not ionized in solution non-conductors

4 STRONG & WEAK ELECTROLYTES STRONG NaCl(s) + H 2 O Na + (aq) + Cl (aq) + H 2 O salt completely ionized HCl(aq) + H 2 O H 3 O + (aq) + Cl (aq) complete ionization of strong acid WEAK NH 3 (aq) + H 2 O NH 4+ (aq) + OH (aq) partial ionization of weak base or weak acid HC 2 H 3 O 2 (aq) + H 2 O H 3 O + (aq) + C 2 H 3 O 2 (aq) NON C 6 H 12 O 6 (s) + H 2 O C 6 H 12 O 6 (aq) + H 2 O glucose no ionization

5 Flowchart for identifying electrolytes note: ionic compounds are strong electrolytes but they could be insoluble(!!!) memorize strong acids and bases (BLB Table 4.2); If a compound is an acid or a base, but NOT one of the strong acids or bases, then it MUST be a weak electrolyte Common misconception: electrolytes are ionic compounds: this is NOT TRUE (e.g., HCl) Sheets Page 3 Lecture 18

6 Which of these aqueous solutions has the greatest total concentration of ions? Which has the least? M NH 4 NO M Pb(NO 3 ) M Na 2 SO M AlPO M C 6 H 12 O 6 (sugar)

7 VAPOR PRESSURE LOWERING Raoult s Law vapor pressure of solution P A = X A P A o mole fraction of solvent vapor pressure pure solvent vapor pressure lowering is a colligative property its depends on the conc but not on the nature of the solute

8 RAOULT S S LAW EXAMPLE 1 Calculate total vapor pressure of a liquid at room temperature that is composed of a mixture of benzene and toluene. The mole fractions of benzene and toluene are X ben = 0.33 and X tol = Benzene: Toluene: o Pben P o tol P A = X A P A o = 75 torr = 22 torr

9 RAOULT S S LAW EXAMPLE 2 Calculate the vapor pressure at 25 o C of a solution made by adding ml of glycerin (C 3 H 8 O 3, a nonvolatile nonelectrolyte with a density of 1.26 g/ml) to g of water. The vapor pressure of pure water at 25 o C is 23.8 torr. P A = X A P A o

10 BOILING POINT ELEVATION FREEZING POINT DEPRESSION Shift in vapor pressure, and shift in phase diagram explains ΔT f and ΔT b ΔT b = K b m molal BP elevation const ΔT f = K f m molal FP depression const molality of the solution (solute particles)

11 FREEZING POINT DEPRESSION SEAWATER Ocean salinity ~ 35 g salt / 1 kg seawater Cl SO 4 2 Mg 2+ Na + Ca 2+ K + molar mass of NaCl is 58.5 g/mol 35 g = 0.60 mol NaCl 58.5 g/mol x mol m = = 1.2 molal 1000 g seawater K f = 1.86 C/m for H 2 O ΔT f = K f m = (1.86)(1.2) = 2.23 C T f = 2.23 C

12 FREEZING POINT DEPRESSION EXAMPLE Choose the solute which would decrease the freezing point to -5 o C of a solution that is made by dissolving moles of the substance in 100 g of water. The freezing point depression constant of water is 1.86 o C/m A. NaF B. CaCl 2 C. Al(NO 3 ) 3 D. Ca 3 (PO 4 ) 2 E. C 6 H 12 O 6

13 OSMOTIC PRESSURE dilute solution concentrated solution semipermeable membrane movement continues until osmotic pressure builds up to stop it π = MRT M is molarity of particles Example: cucumber in brine loses water by osmosis to make pickle

14 Osmosis flow of molecules through a semi-permeable membrane; NET movement of is toward solution with higher solute concentration; movement of solvent continues until osmotic pressure builds up to stop it osmotic pressure (): pressure needed to of a molecule through a membrane = n RT = MRT V is osmotic pressure (what units will this be in???) R is gas constant in (L atm)/(mol K) T is temperature in K M is concentration in molarity (mol/l) what does this equation remind you of??? Sheets Page 10 Lecture 20

15 Consequence of osmotic pressure red blood cells: the cell membrane of red blood cells is a semi-permeable membrane cell in hypertonic soln cell in hypotonic soln Sheets Page 12 Lecture 20

16 OSMOTIC PRESSURE EXAMPLE Honey is 82% sugar by mass. Most of the rest is water. Bacteria love sugar, so why don t they grow in honey? Calculate the osmotic pressure of honey to find out. Osmotic pressure of bacteria ~ 8-30 atm. Composition of sugars: 70% fructose and glucose (C 6 H 12 O 6 ) 30% maltose and sucrose (C 12 H 22 O 11 ) Density of honey: 1.40 g/ml

17 Example A 25 ml aqueous solution containing g of hemoglobin has an osmotic pressure of 4.6 torr at 27 C. What is the molar mass of hemoglobin? ( ) = 4.6torr ( ) V = 25mL T = 300K 1atm 760torr = atm 1L 1000mL = 0.025mL = n V RT n = V RT n = atm 300K n = mol K L atm 0.025L ( ) molar mass of hemoglobin = molar mass of hemoglobin = Answer: g/mol 0.420g mol Sheets Page 13 Lecture 20

18 small true solution molecules COLLOIDS SIZE colloidal suspension nm MILK (fat particles) FOG (water droplets) large mixture particles influenced by gravity river silt Phases mutually insoluble hydrophillic vs. hydrophobic colloids

19 WHY IS THE SKY BLUE? Light passes thru solns without scattering Light passes thru colloidal suspensions with scattering (milk, fog) Tyndall Effect - particles scatter light of λ about the same as their size atoms molecules ~ nm, scatter x-rays colloids up to ~hundreds of nm s (visible light is nm) So colloids scatter light Scatter blue more effectively than red

20 LIGHT SCATTERING or WHY IS THE SKY BLUE? sun sun colloids scatter more blue than red

21 Hydrophilic vs. hydrophobic colloids hydrophilic: water-loving hydrophobic: water-fearing (water-soluble) proteins: hydrophobic core with hydrophilic surface detergents: hydrophobic tail with hydrophilic head Sheets Page 16 Lecture 20