VAPOR PRESSURE OF WATER

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VAPOR PRESSURE OF WATER

• Chemistry 1212 Lab
• By Morey LeFeve

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OBJECTIVE

• Experimentally determine the enthalpy of
  vaporization of water the heat (energy) that
  must be supplied to evaporate 1 mole of water at
  constant pressure.
• - Determine the volume of a gas bubble
  containing air
• water as a function of temperature.
  
• - In closed containers vapor (gas) liquid
• - Note (a) if the air pressure outside the
  container is gt the air pressure inside the
  container container will implode
• (b) if the air pressure outside the
  container is lt
• the air pressure inside the container
  container
• will explode

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Important Things to Remember

• A liquids enthalpy, normal boiling point, and
  volatility intermolecular forces present in the
  liquid ? atoms molecules of any liquid are in
  constant motion
• Equilibrium Vapor Pressure the pressure exerted
  on the walls of a container when equilibrium has
  been established.
• Equilibrium vapor pressure INCREASES as
  temperature increases
• - Vapor Pressure atmospheric pressure

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The Relationship of Vapor Pressure Temperature

• Ideal Gas Law used with low temperatures (below
  5 degrees Celsius) bubble contains ONLY air
• nair Pair V air/RT
• - nair moles of air
• - Pair atmospheric pressure
• - Vair volume of bubble
• - T temperature of bath (K)

• Ideal Gas Law used with temperatures above 5
  degrees Celsius bubble becomes saturated with
  water
• - Amount of air contained in the bubble remains
  constant
• Pair nair RT/Vair

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The Relationship of Vapor Pressure Water
(continued)

• Daltons Law of Partial Pressures this is used
  when temperatures are greater than 5 degrees
  Celsius
• Patm Pair Pwater
• - Patm atmospheric pressure
• - Pair partial pressure of air
• - Pwater vapor pressure of water

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The Relationship of Enthalpy to Vapor Pressure at
Different Temperatures

• Clasius-Clapeyeron Equation
• lnP2 lnP1 - Hvap/R(1/T2-1/T1)
• - P1 P2 vapor pressure of water at T1 T2
• - R Ideal Gas Constant (8.314 J/K x mole)
• - T temperature (Kelvin)
• - - Hvap plot lnP vs 1/T straight line
  with the slope (m) - Hvap/R