Random Reef Chemistry Question of the Day: Air vs Water

[Randy Holmes-Farley]

Random Reef Chemistry Question of the Day

Which has more mass:

1. The water in a 10 gallon reef aquarium
2. The air in a 12 ft x 12 ft x 8 foot (l x w x h) room?

 

[Rollnwthdatide]

Trick question? The same, right?

 

[Rollnwthdatide]

Err, the water by .4 lbs?

Mass of air at STP(lbs) 0.075 * 1152(total cubic feet) = 86.4lbs

Mass of 10gal saltwater 8.68lb/gal*10gal= 86.8lbs

 

[sck90]

Let me try :

1. 10 gal = 37.8 liters = 37.8 kg (for fresh water)
37.8 kg * 1.026 (for reef water) = 38.8 kg

2. 12 ft x 12 ft x 8 foot = 1,152 ft^3 = 32.62 m^3 = 32,620 liters
1 liter of air = 1.29 g (could be calculated with the ideal gas law and the molar wt of the different gases composing air, but I found the value on google)
32,620 L * 1.29 g/L = 42,079.8 g = 42.1 kg

My answer would be that the air in a 12 ft x 12 ft x 8 foot (l x w x h) room has more mass than the water in a 10 gallon reef aquarium.

 

[rhitee93]

I haven't done the calcs on the mass of the "air", but assuming the others did the math right, the air has more mass for the reason that a 10 gallon reef tank won't have 10 gallons of water in it.

 

[Luminous74]

About the same.


Next question: How long does it take for the CO₂ level to double in this closed room with one person?

 

[GSPClown94]

Does the mass of air change with elevation?

 

[rhitee93]

Does the mass of air change with elevation?

The density does
PV=nRt

 

[sck90]

Does the mass of air change with elevation?

I'd say it varies with atmospheric pressure, so all other things being equal, a same volume of air is lighter with higher elevation. Maybe air composition varies slightly also, affecting volumetric weight, but can't say for sure.

 

[Randy Holmes-Farley]

I'd say it varies with atmospheric pressure, so all other things being equal, a same volume of air is lighter with higher elevation. Maybe air composition varies slightly also, affecting volumetric weight, but can't say for sure.

It does change with elevation, barometric pressure, etc. .

Does that change the answer? That's the only important part here.

 

[KStatefan]

Does "air" include the water vapor? Not sure that would change the answer but I was wondered about that in a question the other day about the heat energy brought into a skimmer.

 

[Randy Holmes-Farley]

And the answer is...

1. The water in a 10 gallon reef aquarium
2. The air in a 12 ft x 12 ft x 8 foot (l x w x h) room?


The total mass of water (water only) in a 10 gallon reef aquarium is less than 38 kg. 10 gallons = 37.85 L. Liquid water never has a density higher than 1 kg/L, so the mass cannot be more than 38 kg. It is likely considerably lower due to the usual things in a reef tank (rocks, sand, organisms, pumps, etc.)

The room contains 12 ft x 12 ft x 8 = 1152 cubic feet, or 32,621 L.

The density of air at 50 deg F is 1.246 g/L. At 120 deg F, then density is 1.097 g/L.

Thus the mass of air in the room at one atmosphere is between 40.65 kg and 35.785.
At a typical 70 deg F, then density is 1.199 for a mass of 39.1 kg.

Atmospheric pressure changes due to weather might make this 39.1 kg range from 36.2 kg (during strong low pressure events) to 40.0 kg (during strong high pressure events.

Altitude makes a bigger difference.

39.1 kg of air at sea level will translate to about 26.6 kg at 10,000 feet, where there are some reef tanks.

Thus, the best answer is: "It Depends".

At sea level, the air will usually weigh more except during big storms and tanks with little else but water.

At 10,000 feet elevation, the answer will vary quite a lot.