8.3 Colligative Notes
Transcript of 8.3 Colligative Notes
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Objective :
At the end of this lesson students should be able to:
(a) Define colligative properties.
(b) Describe the colligative properties of a solution:
i. lowering of vapour pressure
ii. boiling point elevation
iii. freezing point depression
iv. osmotic pressure
(c) Perform calculations on colligative properties of
a nonelectrol!te solution containing nonvolatile
solutes.
".# $ %olligative Properties
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colligative properties
&'O*D+
boiling point elevation
freezing point depression
vapor pressure lowering
osmotic pressure
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%O,,-A/-0' P*OP'*/-'+
Properties depend onl! on the number of
solute particles in the solution and not their chemical identit!
Colligative means
1collective2
/here are 3 colligative properties4
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%O,,-A/-0' P*OP'*/-'+ O5 6O60O,A/l,'
6O6','%/*O,/'+ +O,7/-O6+
0apor Pressure ,owering
8oiling Point 'levation
5reezing point Depression
Osmotic Pressure
T b 9 K b m
T f 9 K f m
9 MRT
P 9 X soluteP solvento
P solution 9 X solvent P solvent o
or
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6O60O,A/-,' +O,7/'+
;ave negligible vapor pressure
'
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0APO* P*'++7*' ,O'*-6 ( P )
0apor pressure of a solution of a nonvolatile is alwa!s lower than the vapor pressure of
pure solvent
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Pure solvent +olution 9 +olvent > +olute
Present of solute molecules hinder the escape of
solvent molecules at or near the surface into vapor
6umber of solvent molecules in the vapor reduced
0apor pressure of the solution lower than that of
pure solvent
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P solution 9 X solvent P solvent o
According to *aoult?s ,aw@ the vapor pressure
of a solution containing a nonvolatile solute
is eual to the vapor pressure of pure solvent times the mole fraction of solvent
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Another method of calculation:
P solvent 9 X solvent P solvent o
X solvent > X solute 9 B /hus@ X solvent 9 B $ X solute
P solvent 9 X solvent P solvent o
P solvent 9 (B $ X solute) P solvent o
P solvent 9 P solvent $ ( X solute P solvent)o o
P solvent $ P solvent 9 ( X solute P solvent)o o
P 9 X solute P solvent
o
According to *aoult?s ,aw :
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%arbon tetrachloride (%%l3) has a vapor pressureof BCC torr at #o%. /his solvent can dissolves
candle wa@ which is essentiall! nonvolatile.
=olecular formula of candle wa 9 %;3E
hat is the vapor pressure at #o% of a solutionprepared b! dissolving BC.C g of wa in
3C.C g of %%l3F
EXAMPLE
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=ole of wa 9
(solute) (%;3E)
BC.C g
#BC gGmol9 C.C## mol wa
=ole of %%l3 9
(solvent)
3C.C g
BH3 gGmol9 C.EC mol %%l3
=ol of %%l3
/otal mol (wa > %%l3)
C.EC mol
(C.C## > C.EC) mol
9 C.""I
9
X %%l3
9
Ans: EXAMPLE
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+olvent 9 %%l3
P solution 9 X solvent P solvent o
9 C.""I BCC torr
9 "".I torr
P solution X P o
%%l3 %%l39
X %%l3 9 C.""I
According to *aoult?s ,aw:
Ans: EXAMPLE
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8O-,-6 PO-6/ ','0A/-O6 ( T b)
A solution boils at
higher temperature than
the pure solvent
T b (boiling point) is the temperature when
P solution 9 P atmosphere
;igher temperature needed to raise the
vapor pressure of the solution (P solution)
to eual the eternal pressure (P atmosphere)
P solution
J P solvent
o
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/OP-% " : P;A+' 'K7-8,-8*-A
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T b is proportional to the concentration of
of solute particles:
T b m
T b 9 K bm
K b: molal boiling point elevation constant
7nit: o%Gm o% &g mol $B
m: the solution molalit!
7nit: molal
9 T b(solution) $ T b(pure solvent) T b L C
***K b solution is given
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C.CB moles of sucrose dissolved in BCC g water.hat is the boiling point of the aueous solution
at B atmF K b for water is C.HBo%Gm.
EXAMPLE
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=olalit! of
sucrose solution
=ol of sucrose
&g of water
9 C.CB mol C.B &g
9 C.B m
T b 9 K bm
9 C.HB o%Gm C.B m 9 C.CHB o%
9
6onvolatile solute 9 sucrose +olvent 9 water
Ans: EXAMPLE
T b(solution) 9 T b(pure water) > T b
9 (BCC > C.CHB) o% 9 BCC.CHB o%
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5*''M-6 PO-6/ D'P*'++-O6 ( T f )
/he addition of solute reduce number of
solvent molecules that can enter the solid
phase
/he d!namic euilibrium disrupted and can
be re$established onl! at lower temperature
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/OP-% " : P;A+' 'K7-8,-8*-A
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T f is proportional to the concentration of
of solute particles:
T f m
T f 9 K f m
K f
: molal freezing point depression constant
7nit: o%Gm o% &g mol $B
m: the solution molalit!
7nit: molal
9 T f(pure solvent) $ T f(solution) T f L C
***K f solution is given
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hat is the freezing point of a solution
containing 3N" g of eth!lene gl!col
(antifreeze) in #C g of waterF /he molar
mass of eth!lene gl!col is E.CB g.
K f of water 9 B."Eo%Gm
EXAMPLE
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=ol of eth!lene gl!col 9=ass
=olar mass
9
3N" g
E.CB gGmol
9 N.NC" mol eth!lene gl!col
=olalit! of solution 9 =ol of eth!lene gl!col &g of water
9N.NC" mol
#.C &g9 .3CN m
+olute: eth!lene gl!col+olvent: water
Ans: EXAMPLE
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T f 9 K f m
9 B."E o%Gm .3CN m
9 3.3" o%
T f(solution) 9 T f(pure water) $ T f
9 (C.CC $ 3.3") o%
9 $ 3.3" o%
Ans: EXAMPLE
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Applies onl! to aueous solution
O+=O+-+
/wo solutions of different concentrations are
separated b! a semipermeable membrane
/he membrane allows water (solvent)@ butnot solute@ to pass through
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dilutemore
concentrated
/he net flow of water :
from dilute to more concentrated solution
increases the volume of the solution
decreases its concentration
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/OP-% " : P;A+' 'K7-8,-8*-A
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/he applied pressure reuired to stop osmosis
O+=O/-% P*'++7*' ( )
Prevent volumes changes
π M (molarit! of solution)
π mol of solute
volume of solution
π 9 MRT
R : gas constant
T : absolute temperature (in &)
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A ver! diluted solution@ C.CCBC M sugar in water@is separated from pure water b! an osmotic
membrane.
hat osmotic pressure in torr develop Ho%F
EXAMPLE
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π 9 MRT
T 9 (N# > H) &
9 C.CCBC mol, $B C.C"CE , atm mol $B& $B I" &
9 C.C3H atmNEC torr
B atm
9 B".E torr
9 I" &
π
+olute: sugar +olvent: water
Ans: EXAMPLE
/he effect of a nonvolatile solute on
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/he effect of a nonvolatile solute on
the phase diagram for water.
hen a non volatile solute is added to pure solvent@ the
mole fraction decreases. Addition of a solute at constant /and P lowers the solvent chemical potential.