# Vapor pressure example | Chemistry | Khan Academy | Summary and Q&A

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October 18, 2010
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Vapor pressure example | Chemistry | Khan Academy

## TL;DR

Only a portion of the water will evaporate at 25 degrees Celsius due to the equilibrium vapor pressure.

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### Q: What is vapor pressure?

Vapor pressure is the pressure at which the liquid and vapor states of a substance are in equilibrium. It represents the tendency of molecules to escape from the liquid state into the vapor or gaseous state.

### Q: How is the Ideal Gas Law used in this analysis?

The Ideal Gas Law, which states that pressure times volume equals the number of moles of gas times the universal gas constant times temperature, is used to calculate the number of water molecules that need to evaporate at a given temperature and pressure.

### Q: What is the significance of the 23.8 millimeters of mercury vapor pressure?

The vapor pressure of 23.8 millimeters of mercury indicates the pressure at which the liquid and vapor states of water are in equilibrium at 25 degrees Celsius. This value is crucial in determining the number of water molecules that need to evaporate.

### Q: Will all of the water in the container evaporate at 25 degrees Celsius?

No, all of the water will not evaporate. Only a little bit less than half of the water, approximately 983 milliliters, will evaporate to reach the equilibrium vapor pressure.

## Summary & Key Takeaways

• The content discusses the process of water evaporation at 25 degrees Celsius and introduces the concept of vapor pressure.

• The Ideal Gas Law is applied to calculate the number of water molecules that need to evaporate to reach the equilibrium vapor pressure.

• The calculated number of moles of water is then converted to grams and milliliters to determine the amount of water that will evaporate.