VAPOR PRESSURE OF WATER

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Experiment #2. VAPOR PRESSURE OF WATER. What are we doing in this experiment?. Determine the vapor pressure of water at different temperatures. . 2. Determine the heat of vaporization of water. Review of Gas laws. Boyle’s Law:. Charle’s Law:. Avogadros’s Law:. IDEAL GAS LAW.

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Presentation Transcript

Experiment #2

VAPOR PRESSURE OF WATER

What are we doing in

this experiment?

• Determine the vapor pressure of water
• at different temperatures.

2. Determine the heat of vaporization of water.

Review of Gas laws

Boyle’s Law:

Charle’s Law:

Law:

IDEAL GAS LAW

IDEAL GAS LAW

Units of P: atmospheres

Units of V: Liters

Units of T: kelvin

Units of R:

Dalton’s law of partial pressure

A + B + C

Ptotal= PA + PB + PC

Relation between mole fraction and

partial pressure of a gas

A + B

Ptotal= PA + PB

What is

VAPOR PRESSURE OF WATER?

It is the pressure (technically, partial pressure)

exerted by the water molecules in the vapor

phase (gas, water vapor) above the surface

of a liquid at equilibrium at that temperature.

Pvap

Vapor phase

or

Water vapor

Liquid phase

Heat

Define Pressure

It is defined as force applied per unit area

Units of pressure:

H

H

O--H-O-H - - O

H

H

Why does water not vaporize

by itself quickly?

Atmosphere

Vapor phase

or

Water vapor

Liquid phase

Strong inter molecular hydrogen bonds

When does a liquid boil?

Vapor pressure of the liquid = surrounding

atmospheric pressure

What is boiling point of a liquid ?

It is the temperature at which,

Vapor pressure of the liquid = surrounding

atmospheric pressure

Normal boiling point: If Patm= 1.0 atm

Composition of Dry Air

Gas

% by volume

N2

O2

Ar

CO2

Rest

78.09

20.94

0.93

0.03

0.01

What does it mean if a liquid

has high vapor pressure?

• More molecules are found in the
• vapor phase.

2. Does not require a lot of energy to

vaporize the liquid.

3. Weak inter molecular attractive forces

holding the molecules together in their

liquid state.

A comparison

Closed system

Open system

Heat

Heat

Rate of vaporization can equal

rate of condensation

Rate of vaporization cannot equal

rate of condensation

Dynamic equilibrium not

reachable

Dynamic equilibrium

reachable

A comparison

Closed system

Open system

Heat

Heat

As the temperature is increased,

We can send more and more

molecules in to

Vapor phase

There can only be a fixed

number of molecules that

can be in

the vapor phase

A comparison

Closed system

Open system

Heat

Heat

The maximum vapor pressure

that can be reached here is

atmoshpheric pressure

The maximum vapor pressure

that can be reached here is

more than the atmoshpheric

pressure

What is

VAPOR PRESSURE OF WATER?

It is the pressure (technically, partial pressure)

exerted by the water molecules in the vapor

phase (gas, water vapor) above the surface

of a liquid at equilibrium at that temperature.

Pvap

Vapor phase

or

Water vapor

Liquid phase

Heat

EXPERIMENTAL SETUP

Surface of water should cover test tube.

Magnetic stir-bar (position carefully)

Air bubble at the top of a inverted test tube

Stirrer/hot plate

0

1

2

3

4

5

6

7

8

Paste the scale on a 100 mm

test tube as shown, with the

scale facing inside of the tube

Cut the scale out from the

manual

Note:

This scale is unit less and does not correspond to inches or centimeters.

EXPERIMENTAL SETUP

Surface of water should cover test tube.

Magnetic stir-bar (position carefully)

Air bubble at the top of a inverted test tube

Stirrer/hot plate

What are we going to do?

• Measure the volume of the bubble at
• different temperatures

Charle’s Law

Low temperature

Higher temperature

As temperature increase, the volume increases (at constant P,n)

What are we going to do?

2. Calculate the pressure contribution from N2 and O2

in the bubble using the ideal gas equation.

3. Calculate the vapor pressure of water.

Ptotal= PN2 + PO2+ Pwvap

PN2 and PO2 are much greater than Pwvap

Ptotal= PNO+ Pwvap

But,

Ptotal inside the bubble = Patmosphere

So, we have

Patmosphere= PNO+ Pwvap

Pwvap= Patmosphere- PNO

How to convert the change in

the number of divisions to

change in volume?

Have to find out the relationship

between the number of divisions

to number of mL

Vi

Vo

S=4.5

Burrette

Filled with

water

VN

Unscaled

Volume, Vu=vo-vi

DATA

Please use 3 sig. figs., and scientific notation

HEAT OF VAPORIZATION

DEFINITION

It is defined as the heat required to vaporize

(converting a liquid to gas) one mole of a

substance at constant pressure and temperature.

The heat of vaporization is expressed in kJ/mol. `

vap

HEAT OF VAPORIZATION

Clausius-Clapeyron equation

R=8.314 J/K mol

intercept

LnPwvap= m(1/T) + Z

1/Tk, (k-1)

X1

x2

y

Equation of the best-fit line:

1

x

Y=mX + z

Y1

LnPwvap (unitless)

Best-fit line

m = slope =

2

x

Y2

x

R=8.314 J/K mol