Dilute Solution

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Theory of Dilute Solution

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Solutions

Colligative Properties

Dilute solution containing non-volatile 

solutes exhibit the following properties1.Lowering of the vapour pressure

2.Evevation of the boiling point

3.Depression of the freezing point

.!s"otic pressure

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Solutions

Lowering of vapour pressure# $%!&L'()(

L%*

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Solutions

Lowering of vapour pressure#

$%!&L'()( L%*

P % + X %P °%

where

,   X % is the "ole fraction of co"poun %

,   P °% is the nor"al vapor pressure of % at that te"perature

NOTE:  'his is one of those ti"es when ou want to "a/e sure ou have the

vapor pressure of the solvent.

0ole raction  X 

moles of A

total moles in solution X 

 A =

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Solutions

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Positive deviations from Raoult's

Law

%ssu"e that we have two

particel4 % an 5 are "ixe

%ttraction of particel % an %are greater than % an 5

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Solutions

negative deviations from Raoult's

Law

%ttraction of particel % an 5

are greater than % an %

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Solutions

0easure"ent of lowering of vapour

pressure

1. baro"etric "etho

2. "ano"etric "etho

3. !swal an *al/ers na"ic

"etho

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Solutions

5aro"eter "etho

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0ano"eter "etho

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!swal an *al/ers na"ic

"etho

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5oiling pointelevation

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*hen a li6ui is heate4 its vapour

pressure rises an when it e6uals the

at"ospheric pressure the li6ui boils.'he aition of non-volatile solute

lowers the vapour pressure conse6uentl

elevates the boiling point as the solution

has to be heate a higher te"perature to"a/e its vapoure pressure beco"e e6ual

to at"ospheric pressure.

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Solutions

7f

'b boiling point of the solvent'boiling point of the solution

8'elevation of boiling point

so

8'+'-'b

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Solutions

AB = AD

AC AE

  T1 = P-P1

T1-Tb P-P2

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Deter"ination of "olecular

"ass fro" elevation ofboiling point(ince P is constant for the sa"e solvent at afixe te"perature4 fro" 1 we can write

  8'+ P-P1  P

5ut fro" $aoult)s law for ilute solution

P-P19 w0

  P *"(ince 0"ol "ass of solvent is constant

P-P19 w

  P *"

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Solutions

8'9 w x 1

  " *8'+ :b x w x 1

  " *

:b constant

:b+8' x :g-solvent

  "ol solute

:b + $'b2

1;;; x Ltherefore :b+ ;.<2 :=:g

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0easure"ent of boiling point

elevation1. Lanssberger-*al/er-0etho

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2. Cottrel)s 0etho

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0olecular view of $aoult)s law#

reezing point epression

Depens on the solute onl being in the li6ui phase ewer water "olecules at surface# rate of freezing rops

7ce turns into li6ui

Lower te"perature to regain balance

Depression of freezing point

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Solutions

!s"otic pressure

(olvent passes into "ore concsolution increasing its volu"e

'he passage of the solvent canbe prevente b application ofa pressure

'he pressure to preventtransport is the osmotic pressure

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Solutions

Calculating os"otic pressure

'he ieal gas law states

5ut n=> + 0 an so

*here 0 is the "olar concentration of

particles an Π  is the os"oticpressure

?ote# molarity  is use not molality 

nRT  PV   =

 MRT =Π

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Solutions

!s"otic pressure an

"olecular "ass0olar "ass can be co"pute

fro" an of the colligative

properties

!s"otic pressure provies the

"ost accurate eter"ination

because of the "agnitue of Π  ;.;2;; 0 solution of glucose exerts an os"otic pressure of

3@.2 "" Ag but a freezing point epression of onl ;.;2BC

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Solutions

Beckmann’s method

• % ver sensitive ther"o"eter4 calle a Be!mannt"ermometer  is use4 which has a large bulb an onl sixegrees on the whole scale

• (olvent are weighe into the tube %

• 'he tube % is fitte through a cor/ into a large test-tube 54which serves as an air-ac/et

• 'he tube 5 is supporte in a freezing "ixture containe inthe large ar C

• 'he "ercur falls steail to a certain point4 when thesolvent is slightl supercoole. reezing then co""ences4the te"perature at once runs up to the freezing point4 anafterwars re"ains stationar

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Solutions

Rast’s Camphor method

• % /nown weight of ca"phor is ta/en an powere finel

• 'o this a /nown weight of soli solute is ae

•

'he "ixture is "elte to for" a ho"ogeneous solution4 coole4 anpowere r

• 'he "ixture is ta/en in a capillar tube whose one en is seale

•

7ts "elting point is eter"ine carefull b the conventional "elting pointeter"ination "etho

• 'he "elting point of the pure sa"ple of ca"phor is eter"ine separatelusing another capillar tube. 'he ifference in the "elting points gives 8'f 

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%pplication of 5ec/"ann

$ast Cha"por 0etho

" + 0olecular weight of solute

 ∆T   + reezing=0elting point epression

K  f   + Constantsw + *eight of solute

* + *eight of solvent

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Solutions

Colligative Properties of

Electroltes The colligative properties of solution depend

solely on the total number of solute particlespresent in solution. The various electrolytes

ionize and yield more than one particle performula unit in solutin. Therefore the colligativeeect of an electrolyte solution is always greaterthan that of nonelectrolyte of the same molalconcentration.

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!an’t "o #actor

!an’t "o #actor $i% is de&ned as the

ratio of the colligative eect producedby an electrolyte solution to thecorresponding eect for the sameconcentration of nonelectrolytesolution.

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$elation between >an)t Aoff

actor an Degree of Dissociation

'uppose ( mole of an electrolyte iscapable of forming n ions on completedissociation. )nd α  is degree ofdissociation* then +

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Solutions

,umber of undissociatedmoleculs - (α 

,umber of ions produced - α n

 Total number of particles - (α α n

"ence*

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)ctivity Coe/cient0., 1ewis discovered that the

e2perimentally determined value ofconcentration whether of moleculesor ions in solution is less than theactual concentration. The apparent

value of concentration is termedactivity* de&ned as the ratio betweenthe activity and actual concentrationof the molecul or ion in solution