Vapor Pressure and Boiling Point

Measuring how vapor pressure changes with temperature

Gas Laws Advanced (University) 45 min ~$12.00

Objective

Measure the vapor pressure of water at different temperatures and verify the Clausius-Clapeyron equation.

Background

The vapor pressure of a liquid increases exponentially with temperature. By measuring the temperature at which water boils under different reduced pressures (or measuring vapor pressure at different temperatures), the enthalpy of vaporization can be determined from the Clausius-Clapeyron equation.

Safety Warnings

  • Handle vacuum apparatus carefully — implosion risk with glass
  • Hot water can cause burns
  • Use proper vacuum-rated glassware only

Required PPE

goggles lab_coat

Materials

  • Distilled water (200 mL)
  • Ice (500 g)
    For cold water bath

Equipment

Round-bottom flask (500 mL) Vacuum pump or syringe Pressure gauge or manometer Thermometer Hot plate Rubber tubing Graph paper

Procedure

1

Set up the flask with water, thermometer, and pressure gauge. Connect to the vacuum pump.

5 min Use vacuum-rated glassware only
2

Heat the water to 80°C. Record the temperature and vapor pressure.

5 min
3

Reduce pressure with the vacuum pump. Note the temperature at which boiling begins.

5 min
4

Record at least 8 temperature-pressure data points from 40°C to 100°C.

15 min
5

Plot ln(P) vs 1/T(K). The slope = -ΔHvap/R.

10 min
6

Calculate ΔHvap from the slope. Compare with the literature value of 40.7 kJ/mol.

5 min

Expected Results

The ln(P) vs 1/T plot should be linear with a negative slope. The calculated ΔHvap should be approximately 40-44 kJ/mol, close to the accepted value of 40.7 kJ/mol for water.

Cleanup

Release vacuum slowly. Turn off heater. Drain water and dry apparatus.

Details

Category
Gas Laws
Difficulty
Advanced (University)
Duration
45 min
Est. Cost
$12.00
Steps
6
Materials
2

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Frequently Asked Questions

What is the objective of Vapor Pressure and Boiling Point?
Measure the vapor pressure of water at different temperatures and verify the Clausius-Clapeyron equation.
How difficult is Vapor Pressure and Boiling Point?
This experiment is rated as Advanced (University). It takes approximately 45 minutes to complete.
What safety precautions are needed for Vapor Pressure and Boiling Point?
Key safety precautions include: Handle vacuum apparatus carefully — implosion risk with glass; Hot water can cause burns; Use proper vacuum-rated glassware only.
What materials are needed for Vapor Pressure and Boiling Point?
The main materials required are: Distilled water, Ice.
What results should I expect from Vapor Pressure and Boiling Point?
The ln(P) vs 1/T plot should be linear with a negative slope. The calculated ΔHvap should be approximately 40-44 kJ/mol, close to the accepted value of 40.7 kJ/mol for water.