Vapor Pressure Powerpoint

8/10/2019 Vapor Pressure Powerpoint

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Vapor Pressure



(a) Initially, no molecules of the liquid in the vapor phase (zerovapor pressure)

(b) On heating, a liquid phase is formed and vaporization begins.

At equilibrium, no. of molecules leaving the liquid is equal to the no. of molecules returning to liquid phase. These equal

rate produce stable vapor pressure that does not change as

long as the temperature remains constant



• At any temperature some molecules in a liquid have enough

energy to escape and enter the vapor phase.

• As the temperature rises, the fraction of molecules that have

enough energy to escape increases.



• The rate of evaporation is constant at any given temperature.

• The rate of condensation increases when concentration of

molecules in vapor phase increases.

•

A state of dynamic equi l ibr ium ( rate of forward process isexactly balanced by rate of reverse process ) is reached when

rates of condensation = evaporation.

• Therefore, the vapor pressure is the pressure exerted by its

vapor when the liquid and vapor states are in dynamic

equilibrium.



Vapor pressure of solution

• The vapor pressure of a solvent above a solution is lower than

the vapor pressure of pure solvent.

-The solute particles replace some of the solvent molecules at the surface.

•

A concentrated solution will draw pure solvent vapor into itdue to tendency to mix.

• Results in reduction in vapor pressure.



Vapor pressure lowering

• The vapor pressure of the solution is directly proportional to

the amount of the solvent in the solution.

• The difference between the vapor pressure of the pure solvent

and the vapor pressure of solvent in solution is called vapor

pressure lowering.



Raoult’s Law for non-volatile solute

• The vapor pressure of the solvent above a solution containing

a non-volatile solute (a solute that does not have a vapor

pressure of its own) is directly proportional to the mole

fraction of solvent in the solution. This behavior is summed up

in Raoult' s Law :

Psolvent

= Xsolvent

Posolvent

where:

Psolvent

is the vapor pressure of the solvent above the solution,

Xsolvent

is the mole fraction of the solvent in the solution

Posolvent

is the vapor pressure of the pure solvent.



1) Calculate the vapor pressure of H20 in a solution prepared by

mixing 99.5g of C12H22O11 with 300ml of H2O.

99.5g C12H22O11 × 1 2211

342.30 122211 = 0.2907 mol C12H22O11

300 ml H2O ×1.00

1 ×

1 20

18.02 20 = 16.65 mol H2O

= 16.65 200.2907 122211:16.65 2

= 0.9828

PH2O = H2O . P°H2O

= 0.9828 × 23.8 torr= 23.4 torr #



Raoult’s Law + Dalton’s Partial Pressure Law for

volatile solute

• A volatile solute (a solute that has a vapor pressure of its own)

will contribute to the vapor pressure above a solution in which

it is dissolved.

• The total vapor pressure above a solution will be the sum of

the vapor pressure of solute and solvent.

Psolvent = XsolventPosolvent

Psolute

= Xsolute

Po

solute Ptotal = Psolute + Psolvent



2) Calculate the component and total vapor pressure of a solution

prepared by mixing 3.95g of CS2 with 2.43g of C3H6O.

(Vapor pressure of CS2 = 515 torr, vapor pressure of C3H6O = 332 torr)

3.95g CS2 × 1 2

76.15 2 = 0.05187 mol CS2

2.43g C3H6O × 1 36

58.0 36

= 0.04184 mol C3H6O

CS2 =0.05187 2

0.05187 2:0.04184 36

= 0.5535

PCS2 = CS2 . P° CS2

= 0.5535 × 332 torr

= 285 torr



C3H6O =0.04184 36

0.05187 2:0.04184 36

= 0.4465

PC3H6O = C3H6O . P°C3H6O

= 0.4465 × 332 torr

= 148 torr

Ptotal = 285 torr + 148 torr

= 443 torr #





Nonideal solution

• When solute-solvent interaction are stronger than the solute-solute & solvent-solvent interactions, the total pressure of the

solution will be less than predicted by Raoult’s Law

• When the solute-solvent interaction are weaker than the

solute-solute & solvent-solvent interactions, the total vapor pressure of the solution will be more than predicted by

Raoult’s Law

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Vapor Pressure Powerpoint