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Converters & Calculators
Vapor Pressure: Basics And Examples
What is Vapour Pressure?
The vapour pressure is the pressure exerted by a vapour in equilibrium with its liquid or solid phase at a specified temperature. It indicates the rate at which a substance transitions from liquid to gas.
Factors that Influence Vapour Pressure
Temperature
With increasing temperature, vapour pressure increases because more molecules obtain sufficient energy to leave the liquid phase.
Intermolecular Forces
Stronger intermolecular forces lead to a lower vapour pressure given that the molecules require increased energy to evaporate.
Type of Substance
Depending on the molecular structure and bonding, different substances exhibit varying vapour pressures.
Measurement of Vapour Pressure
- Manometric Method: A sealed container with a liquid is used, and the vapour pressure is measured using a manometer once the system reaches equilibrium.
- Dynamic Method: Changes in the liquid volume or mass are monitored over time to calculate the vapour pressure.
- Effusion Method: The rate at which gas molecules escape through a small aperture is measured, thereby allowing the calculation of the vapour pressure.
- Clausius-Clapeyron Equation: This equation is applied to calculate the vapour pressure at varying temperatures.
The Importance of Vapour Pressure Measurement
Vapour pressure measurement is critical in various fields, including:
- Weather Forecasting: Vapour pressure is used to determine humidity and, consequently, to forecast weather patterns.
- Industrial Processes: It assists in distillation, evaporation, and other processes that involve phase transitions.
- Chemical Process Engineering: It plays a role in the design of devices such as heat exchangers, reactors, and evaporators.
- Pharmaceutical Industry: Vapour pressure measurements are important for the stability and storage of volatile compounds.
Examples of Vapour Pressure
Water
At 25°C, water has a vapour pressure of approximately 23.8 mmHg, indicating moderate volatility.
Acetone
Acetone exhibits a higher vapour pressure than water at the same temperature; consequently, it is more volatile.
Mercury
Mercury shows a very low vapour pressure, which arises from its strong metallic bonds and minimal tendency to evaporate.
Vapour Pressure Table
The table below presents vapour pressure values for various substances at different temperatures. The vapour pressure refers to the pressure exerted by the vapour when it is in equilibrium with its liquid (or solid) phase at a specified temperature.
For quick and simple results, try our Pressure Unit Converter. Visit our Toolbox page for additional useful tools.
|
Substance |
Vapour Pressure at 20°C (mmHg) |
Vapour Pressure at 100°C (mmHg) |
|
Water |
17.5 |
760 |
|
Ethanol |
44.6 |
400 |
|
Acetone |
180 |
760 |
|
Methanol |
95.3 |
1 300 |
|
Benzene |
75.1 |
450 |
|
Diethyl Ether |
430 |
5 800 |
|
Mercury |
0.0012 |
0.2 |
|
Hexane |
150 |
450 |
|
Toluene |
22.3 |
230 |
|
Chloroform |
160 |
500 |
Further information is available at Stanford Advanced Materials (SAM).
Frequently Asked Questions
What is vapour pressure?
Vapour pressure is the pressure exerted by a vapour when it is in equilibrium with its liquid or solid form.
How does temperature affect vapour pressure?
Generally, vapour pressure increases with temperature because a higher number of molecules obtain the energy required to evaporate.
Why does water have a lower vapour pressure than acetone?
Water exhibits stronger hydrogen bonding; therefore, more energy is required for it to evaporate compared to acetone.
Can vapour pressure predict the volatility of a substance?
Yes, a higher vapour pressure indicates greater volatility and a higher propensity to evaporate.
How important is vapour pressure for weather forecasting?
Vapour pressure contributes to humidity, thereby influencing weather conditions and precipitation levels.
Chin Trento
