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COLLIGATIVE PROPERTIESCOLLIGATIVE PROPERTIES
Depend on the number of soluteparticles in solution but not on theidentity of the solute
Vapor pressure loweringBoiling point elevationFreezing point depressionOsmotic pressure
EXAMPLES
0.5 m solution of Pb(NO3)2
0.5 m Pb2+ and 1.0 m NO3– → 1.5 m total ions
0.5 m HC2H3O2 (acetic acid)
HC2H3O2 H+ + C2H3O2–
solution is between 0 and 1.0 m in total ionssolution is between 0.5 and 1.0 m in all species
EXPRESSINGEXPRESSINGCONCENTRATIONCONCENTRATION
weightpercent =
mass componenttotal mass
x 100
mole fractionof component
moles componenttotal moles=
molarity = moles soluteliters solution
molality = moles solutemass solvent (kg)
(%)
(fraction)
(M)
(m)
ELECTROLYTESELECTROLYTESA substance that yields ions whendissolved in water is an electrolyte
Strong electrolytescompletely ionized in solutiongood conductors
Weak electrolytespartially ionized in solutionpoor conductors
Nonelectrolytesnot ionized in solutionnon-conductors
STRONG & WEAKSTRONG & WEAKELECTROLYTESELECTROLYTES
NaCl(s) + H2O → Na+(aq) + Cl–(aq) + H2Osalt completely ionized
HCl(aq) + H2O → H3O+(aq) + Cl–(aq)complete ionization of strong acid
NH3(aq) + H2O NH4+(aq) + OH–(aq)
partial ionization of weak base or weak acid
HC2H3O2(aq) + H2O H3O+(aq) + C2H3O2–(aq)
C6H12O6(s) + H2O → C6H12O6(aq) + H2O
glucose no ionization
STRONGSTRONG
WEAKWEAK
NONNON
Sheets Page 3 Lecture 18
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)
Which of these aqueous solutions hasthe greatest total concentration of ions?Which has the least?
1. 0.4 M NH4NO3
2. 0.2 M Pb(NO3)2
3. 0.3 M Na2SO4
4. 0.2 M AlPO4
5. 0.5 M C6H12O6 (sugar)
VAPOR PRESSUREVAPOR PRESSURELOWERINGLOWERING
Raoult’s Law PA = XA PAo
vapor pressureof solution
vapor pressurepure solvent
mole fractionof solvent
vapor pressure lowering is a colligativeproperty — its depends on the concbut not on the nature of the solute
RAOULTRAOULT’’S LAW EXAMPLE 1S LAW EXAMPLE 1
PA = XA PAo
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 areXben = 0.33 and Xtol = 0.67.
Benzene:
Toluene:
75obenP torr=
22otolP torr=
RAOULTRAOULT’’S LAW EXAMPLE 2S LAW EXAMPLE 2
PA = XA PAo
Calculate the vapor pressure at 25 oC of a solution made by adding 50.00 mL of glycerin (C3H8O3, a nonvolatile nonelectrolyte with a density of 1.26 g/mL) to 500.0 g of water. The vapor pressure of pure water at 25 oC is 23.8 torr.
BOILING POINT ELEVATIONBOILING POINT ELEVATIONFREEZING POINT DEPRESSIONFREEZING POINT DEPRESSION
Shift in vapor pressure, and shiftin phase diagram explains ΔTf and ΔTb
ΔTb = Kb m
ΔTf = Kf m
molal BP elevation const
molal FP depression const
molality of the solution(solute particles)
FREEZING POINT DEPRESSIONFREEZING POINT DEPRESSIONSEAWATERSEAWATER
Ocean salinity ~ 35 g salt / 1 kg seawater
Cl– SO42– Mg2+ Na+ Ca2+ K+
molar mass of NaCl is 58.5 g/mol
35 g58.5 g/mol
= 0.60 mol NaCl
m = 1.2 mol
1000 g seawater = 1.2 molalx 2
Kf = 1.86 °C/m for H2O
ΔTf = Kf m = (1.86)(1.2) = 2.23 °C
Tf = – 2.23 °C
FREEZING POINT DEPRESSIONFREEZING POINT DEPRESSIONEXAMPLEEXAMPLE
Choose the solute which would decrease the freezing point to -5 oC of a solution that is made by dissolving 0.0538 moles of the substance in 100 g of water. The freezing point depression constant of water is 1.86 oC/m
A. NaFB. CaCl2C. Al(NO3)3D. Ca3(PO4)2E. C6H12O6
concentratedsolution
dilutesolution
OSMOTIC PRESSUREOSMOTIC PRESSURE
semipermeablemembrane
movement continues until osmotic pressurebuilds up to stop it
π = MRT M is molarity of particles
Example: cucumber in brine loses waterby osmosis to make pickle
Sheets Page 10 Lecture 20
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
=nV
RT =MRT
• 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 12 Lecture 20
Consequence of osmotic pressure • red blood cells: the cell membrane of red blood cells is a semi-permeable membrane cell in hypertonic sol n
cell in hypotonic sol n
OSMOTIC PRESSUREOSMOTIC PRESSUREEXAMPLEEXAMPLE
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(C6H12O6)
30% maltose and sucrose(C12H22O11)
Density of honey: 1.40 g/mL
Sheets Page 13 Lecture 20
Example A 25 mL aqueous solution containing 0.420 g of hemoglobin has an osmotic pressure of 4.6 torr at 27°C. What is the molar mass of hemoglobin?
Answer: 6.84 104 g/mol
= 4.6torr( )1atm
760torr= 6.05 10 3atm
V = 25mL( )1L
1000mL= 0.025mL
T = 300K
=nV
RT
n =V
RT
n =6.05 10 3atm
300Kmol K
0.08206L atm0.025L( )
n =
molar mass of hemoglobin =0.420g
6.14 10 6molmolar mass of hemoglobin =
COLLOIDSCOLLOIDSsmall large
SIZE
truesolution
colloidalsuspension
particlesinfluencedby gravity
molecules
mixture
2–2000 nm
MILK(fat particles)
FOG(water droplets)
river silt
Phases mutually insolublehydrophillic vs. hydrophobic colloids
WHY IS THE SKY BLUE?WHY IS THE SKY BLUE?
Light passes thru solns without scatteringLight passes thru colloidal suspensions
with scattering (milk, fog)Tyndall Effect - particles scatter light of
λ about the same as their size
atomsmolecules
~ nm, scatter x-rays
colloids up to ~hundreds of nm’s(visible light is 400-700 nm)
So…colloids scatter lightScatter blue more effectively than red
LIGHT SCATTERINGLIGHT SCATTERING……oror……
WHY IS THE SKY BLUE?WHY IS THE SKY BLUE?
sun
sun
colloids scatter more blue than red
Sheets Page 16 Lecture 20
Hydrophilic vs. hydrophobic colloids • hydrophilic: water-loving • hydrophobic: water-fearing • (water-soluble) proteins: hydrophobic core with hydrophilic surface • detergents: hydrophobic tail with hydrophilic head
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