# Water Vapor content at equilibrium in vacuum [duplicate]

Given a hard bodied vacuum sealed container $$C$$. $$C$$ itself is internally divided into two sealed sub-containers $$C_{lower}$$ and $$C_{upper}$$ (each having volume $$V$$ $$m^3$$).

$$C_{lower}$$ is completely filled with water, whereas $$C_{upper}$$ contains (assume perfect) vacuum. Assuming the environmental temperature of $$T$$ degrees Celsius, when we open the separation between both how do we (approximately) calculate the maximum amount of water vapor ($$kg/m^3$$ at temperature $$T$$) (at equilibrium)? If needed, we can assume standard atmospheric pressure to keep things simple.

I am aware about the Absolute Humidity values for air (https://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html) but unclear about this vacuum scenario.

• – Karsten
May 12, 2022 at 16:26
• @KarstenTheis Thank you. Yes agreed its related but that seems to be just a conceptual question. I am unsure how to calculate the actual vapor content from it.. May 12, 2022 at 16:31
• You’d get the equilibrium vapor pressure for temp T which you can read off of an H2O phase diagram May 12, 2022 at 16:33
• As I understand you are referring to this (en.wikipedia.org/wiki/Vapor_pressure_of_water). Just a followup aren't these values for an open system (air at standard 1 atm). Or can I assume these values are for a closed vacuum system (as in Q) and I can use them to calculate the vapor density of above system? May 12, 2022 at 16:38
• The vapor pressure determines the temperature T, or vice versa. When you say you can impose "standard atmospheric pressure" the effect is probably not what you think. The relevant pressure is the vapor pressure. See for instance chemistry.stackexchange.com/questions/138256/… May 12, 2022 at 17:18