# Q & A: water liquid-vapor equilibrium

Q:
Hi, I have a question which has bother me for some times. there is a 50mL glass vial filled with dry air, then i put 50uL water in it and seal the vial with well-fixed lip(it could'not be cracked or leak any gas).My question as follows: 1. if i heat the vial from 25oC to 105oC and hold temperature at 105oC for enough time, is all water will tranfer to head space? 2. How to calculate the total pressure, is it just the sum of air pressure(128.52kPa at 105oC) and Saturated water vapor pressure(120.79)? 3. If the volume of water is not 50uL but 20uL, all of water will transfer to vapor? At last, i look forward to your kindness help and your detailed answer.
- Wilson Hu (age 28)
Guangzhou
A:
1. I'm not sure I understand the question there.

2. Yes, the total pressure will be almost exactly the sum of the pressures for the the air and water separately. Since there will still be just a little liquid left, the water pressure will be the equilibrium vapor pressure.

3. Yes, if there's not enough water to reach the equilibrium vapor pressure, all the water turns to vapor. You can then calculate the water vapor pressure from the ideal gas law, p=nRT/V.

Mike W.

(published on 09/10/2011)

## Follow-Up #1: Saturated water vapor pressure

Q:
Thank you so much for your detailed answer, i wanna ask another question: what is the relationship between the equilibrium vapor pressure and Saturated water vapor pressure. Thanks again:)
- Wilson (age 28)
Guangzhou
A:
I believe those two phrases mean the same thing.

Mike W.

(published on 09/11/2011)