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