Chem 106 Thursday 10 Feb 2011
Colligative Properties overview
Vapor Pressure of Liquid - effect of non-volatile solute (Raoult)
Boiling Point Elevation
Freezing Point Depression
Osmotic Pressure
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Colligative Properties Overview
Colligative mean “holds together”.
It refers to how solutes change the properties of the
liquid based on the concentration of the solute (and
not the shape or type).
2nd phase
(vapor, solid, or
through a
membrane)
solution
solvent
solute
The solute molecules or ions block the surface and “prevent escape” of the
solvent molecules. The type of particle does not matter.
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Effects of non-volatile solutes:
1. Vapor pressure lowering (Raoult’s Law)
2. Boiling point elevation
3. Freezing point depression
4. Osmotic pressure lowering
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Vapor pressure lowering by a non-volatile solute (Raoult’s Law)
0
Psolution  X solvent  Psolvent
vapor pressure of
solution due to the
solvent molecules
=
mole fraction of
solvent
x
vapor pressure
of the pure
solvent
Recall that:
X solvent
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
moles solvent
moles solvent

total moles of solution moles solvent  moles solute
4
Given
Given data
X ether 
0
Pether  X ether  Pether
X ether
X ether

Pether
0
Pether
mol ether
mol ether  mol Chl
Solve for mol Chl, then convert
that to grams using molar mass.
Now solve for mol Chl
𝑋𝑒𝑡ℎ𝑒𝑟 ∗ 𝑚𝑜𝑙 𝑒𝑡ℎ𝑒𝑟 + 𝑋𝑒𝑡ℎ𝑒𝑟 ∗ 𝑚𝑜𝑙 𝐶ℎ𝑙 = 𝑚𝑜𝑙 𝑒𝑡ℎ𝑒𝑟
Pether 456.92 mmHg
 0 
 0.985008
1 − 𝑋𝑒𝑡ℎ𝑒𝑟
Pether 463.57 mmHg
𝑚𝑜𝑙 𝐶ℎ𝑙 = 𝑚𝑜𝑙 𝑒𝑡ℎ𝑒𝑟 ∗
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𝑋𝑒𝑡ℎ𝑒𝑟
5
47.85

 1 - X ether
mol Chl  mol ether 
 X ether
mol Chl  260.8 g Eth ´

÷
÷

1 mol Eth  1 - 0.985008 

÷
74.12 g Eth  0.985008 
mol Chl  0.053554 mol Chl
0.053554 mol Chl ´
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893.5 g Chl
 47.85 g Chl
mol Chl
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Effects of (non-volatile) solutes:
1. Vapor pressure lowering (Raoult’s Law)
2. Boiling point elevation
3. Freezing point depression
4. Osmotic pressure lowering
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Colligative Properties Overview
Colligative mean “holds together”.
It refers to how solutes change the properties of the
liquid based on the concentration of the solute (and
not the shape or type).
2nd phase
(vapor, solid, or
through
membrane)
solution
solvent
solute
The solute molecules or ions block the surface and “prevent escape” of the
solvent molecules. The type of particle does not matter.
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Boiling point elevation by non-volatile solute
HO
Example: “anti-freeze” prevents boilover (and freezing) of engine coolant.
OH
H
H
H
H
ethylene glycol
(toxic)
HO
OH
H
H
H
CH3
propylene glycol
These compounds are used
because they are very soluble in
water, non-corrosive to metal
parts, and relatively
inexpensive.
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Phase diagram of water
Vapor pressure of solution at T = 100 °C is LESS
THAN vapor pressure of pure water.
Solutions still
require P = 1.00 atm
to boil.
P (atm)
1.00
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T (°C)
100 110
So, to get the solution to boil,
you have to heat it hotter.
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Boiling elevation = ΔTbp
ΔTbp = Kbpmsolute
“molal boiling point
elevation constant”
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Boiling point elevation by ionic compounds:
The colligative effect depends on the NUMBER OF
PARTICLES ONLY. So we take this into account by
including the factor “i”.
i = moles of particle per mole of solute
These can be
molecules, or
separate ions of
ionic solute.
i=2
=3
=3
=1
for NaCl
for CaCl2
for H2SO4
for ethylene glycol
ΔTbp = iKbpmsolute
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C
D
B
A
0.082 mol AlCl3 4 mol ions 0.328 mol ions

x
kg
mol AlCl3
kg
0.10 mol CaBr2 3 mol ions 0.30 mol ions

x
kg
mol CaBr2
kg
0.090 mol Al ( NO3 )3
4 mol ions
0.36 mol ions

x
kg
mol Al ( NO3 )3
kg
0.39 mol gluc 1 mol molec 0.39 mol molec

x
kg
mol gluc
kg
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Which aqueous solution (each 0.50 m) has the
highest boiling point?
Ethylene glycol
Chromium (III) nitrite
Sodium chloride
Calcium sulfite
23
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1.
2.
3.
4.
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Which aqueous solution (each 0.50 m) has the
highest boiling point?
1.
2.
3.
4.
Ethylene glycol I = 1
Chromium (III) nitrite Cr(NO2)3
Sodium chlorate NaClO3 i = 2
Calcium sulfite CaSO3 i = 2
i =4
(Please review ion charge, name, and formula.)
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Effects of (non-volatile) solutes:
1. Vapor pressure lowering (Raoult’s Law)
2. Boiling point elevation
3. Freezing point depression
4. Osmotic pressure lowering
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Solute molecules or ions BLOCK access to the solid surface
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Phase diagram of water & solutions – melting point depression
Must cool to -4°C
(or whatever) to get
ice again.
Pure water + ice @0.0°C/1 atm
Solution (no ice) @0.0°C/1 atm
P (atm)
1.00
New triple point @lower temp/pressure
0.0
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T (°C) ->
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Phase diagram of water and solutions – melting point depression
Liquid phase for pure liquid
Liquid phase for solution
P (atm)
1.00
0.0
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T (°C) ->
21
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Ice crystals in seawater are nearly fresh.
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Ice metamorphoses over a period of weeks and months, which
eliminates salt impurities within the crystals.
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Arctic explorers say that “one-year ice is good for nothing, two-year ice is OK to cook
with and only three-year ice will do for tea”.
http://www.jeanlouisetienne.fr/poleairship/EN/images/encyclo/imprimer/14.htm
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Freezing Point depression elevation = ΔTfp
ΔTfp = Kfpmsolute
“molal freezing
point depression
constant”
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Melting point depression by ionic compounds:
The colligative effect depends on the NUMBER OF
PARTICLES ONLY. So we take this into account by
including the factor “i”.
i = moles of particle per mole of solute
These can be
molecules, or
separate ions of
ionic solute.
i=2
=2
=3
=1
for CsI
for HCl
for Ca(ClO4)2
for sucrose
ΔTfp = iKfpmsolute
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Which aqueous solution (each 0.50 m) has the
lowest freezing point?
Sodium bromide
Sodium chlorate
Calcium chlorate
Cobalt (III) nitrate
37
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2
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1.
2.
3.
4.
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Which aqueous solution (each 0.50 m) has the
lowest freezing point?
1.
2.
3.
4.
Sodium bromide NaBr i = 2
Sodium chlorate NaClO3 i = 2
Calcium chlorate Ca(ClO3)2 i = 3
Cobalt (III) nitrate Co(NO3)3 i = 4
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i=1
62.1
62.1
 g/mol
 T fp  K fp m solute
m solute

m solute 
 T fp
K fp
 T fp
K
fp

- 1 . 155 o C
- 1 . 86 C / m
o
 0 . 6210 m
0.6210 mol solute
x

1kg H 2O
kg solvent
293.0 g H 2O ´
1000 g H 2O
0.6210 mol solute
´ 0.2930kg H 2O  0.18194 mol
x
kg solvent
msolute 
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M
g
11.30 g

 62.12 g / mol  62.1 g / mol
mol 0.18194 mol
30
Which one of the following is the most reasonable
formula for this compound?
C5H2 Not enough H’s
C4H14 Too many H’s
C 2H 6O 2
C 3H 6O
C3H8O Formula wrong for 62 g/mol
13
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C3
H8
O
C4
H1
4
C3
H6
O
11
10
C5
H2
1.
2.
3.
4.
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This is the formula for ethylene glycol,
which is possible to synthesize by rather
easy methods in the lab, in contrast to
the molecule shown below.
3. C3H6O
C 2H 6O 2
Turns out that C5H2 does have at least one energy
minimum as calculated by the PM3 semi-empirical
method. This molecule might exist in the gas
phase at close to Absolute Zero out in space
somewhere…
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Effects of (non-volatile) solutes:
1. Vapor pressure lowering (Raoult’s Law)
2. Boiling point elevation
3. Freezing point depression
4. Osmotic pressure lowering
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Osmotic pressure = extra pressure that must be applied to the SOLUTION
SIDE of semi-permeable membrane to EQUALIZE solvent flow.
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OSMOSIS = diffusion through a semi-permeable membrane.
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OSMOTIC Pressure =  (atm)
 = iMsoluteRT
i = Moles of
particles per mole
of solute
Molarity of solute
Absolute Temperature (Kelvin, K)
Gas Constant = 0.08206 L-atm/(K-mol)
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