September 23, 2019, 02:07:03 PM
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Topic: Finding mass of water at a given humidity  (Read 160 times)

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Offline Kolbrandir

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Finding mass of water at a given humidity
« on: September 13, 2019, 10:07:03 PM »
Okay maybe this isn’t quite the best place for this. I’m a third year Chemistry student in university. I have a physical chemistry course, and I had this homework question that we both didn’t do in class and I can’t figure out if it’s done correctly.

There is a 400m^3 room at 27 C. The relative humidity is 60%. Find the mass of water in the room.

I know that 60% humidity means that the air contains 60% of the maximum water content the air would have if it was saturated (in equilibrium with liquid water) at that defined temperature. Thing is I have no idea how to find the partial pressure of water vapour if the air is saturated. Otherwise the question would be quite simple.

What I did, was I used this book of steam tables (which hasn’t been mentioned in class yet), looked at 27°C, and there’s a “saturation volume” of 38.774 m^3/kg. I used that to get a mass of water of 6.19kg in the room.

Is that right/okay to do? Is there some formula I’m missing that will give me partial pressure of water vapour at saturation?

Offline Borek

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Re: Finding mass of water at a given humidity
« Reply #1 on: September 14, 2019, 01:07:10 PM »
Approach looks OK to me (haven't looked at the number itself).

You could use Antoine equation, but it will give you more or less the same number as the steam tables do. Relative humidity is always relative to the saturated vapor, so saturated vapor is a reference point and/or best starting point for the rest of calculations.
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Offline Kolbrandir

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Re: Finding mass of water at a given humidity
« Reply #2 on: September 14, 2019, 05:35:33 PM »
Thanks!

I haven’t seen the Antoine equation, but I could take a look. And yeah, I know the relative humidity is just related to saturated vapour - hence why I tried to find it. My friend got the same mass via a different route, so I should be good.

Offline Enthalpy

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Re: Finding mass of water at a given humidity
« Reply #3 on: September 16, 2019, 02:03:31 PM »

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