VAPOR PRESSURE OF WATER

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Experiment 2
VAPOR PRESSURE OF WATER
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What are we doing in this experiment?

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

2. Determine the heat of vaporization of
water.
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Review of Gas laws
Boyles Law
Charles Law
Avogadross Law
IDEAL GAS LAW
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IDEAL GAS LAW
Units of P atmospheres
Units of V Liters
Units of T kelvin
Units of R
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Daltons law of partial pressure
A B C
Ptotal PA PB PC
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Relation between mole fraction and partial
pressure of a gas
A B
Ptotal PA PB
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(No Transcript)
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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
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Define Pressure
It is defined as force applied per unit area
Units of pressure
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Why does water not vaporize by itself quickly?
Atmosphere
Vapor phase or Water vapor
Liquid phase
Strong inter molecular hydrogen bonds
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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
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Composition of Dry Air
Gas
by volume
N2 O2 Ar CO2 Rest
78.09 20.94 0.93 0.03 0.01
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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.
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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
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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
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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
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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
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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
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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.
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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
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What are we going to do?

• Measure the volume of the bubble at
• different temperatures

Charles Law
Low temperature
Higher temperature
As temperature increase, the volume increases (at
constant P,n)
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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
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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
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Vi
Vo
S4.5
Burrette Filled with water
VN
Unscaled Volume, Vuvo-vi
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DATA
Water Temp (oC) T Scale Value S Water Temp (K) TK Bobble Vol. (mL) VB Press. N2 O2 (atm) PNO Water Vap P (atm) PW Vap Ln PW Vap 1/TK





Please use 3 sig. figs., and scientific notation
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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
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HEAT OF VAPORIZATION
Clausius-Clapeyron equation
R8.314 J/K mol
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intercept
LnPwvap m(1/T) Z
1/Tk, (k-1)
X1
x2
y
Equation of the best-fit line
1
x
YmX z
Y1
LnPwvap (unitless)
Best-fit line
m slope
2
x
Y2
x
R8.314 J/K mol