How much weight will one cubic foot of air hold up in water?

Question

Assume the container holding the air weighs 0 and it is at sea level in fresh water. I am wanting to find out how much weight these two tubes filled with air will support.

Answer 1

Air is a mixture of gasses....mostly Nitrogen gas (80%) and Oxygen gas (20%).
The molar mass of N2 is 14.01 g/mol.
The molar mass of O2 is 16.00 g/mol.
The weighted average molar mass of air (80%-20%) is 14.41 g/mol.

1 ft^3 = about 28.32 Liters.

At STP, 1 mole of an ideal gas has a volume of 22.4 Liters. We will assume air acts as an ideal gas under the conditions of the experiment....we full further assume that the conditions of the experiment are at STP.
Thus,
1 ft^3 of an idea gas relates to about 1.26 moles of the gas.

A ft^3 of air is about 1.26 moles of air, with a molar mass of 14.41 g/mol.
Therefore, 1 ft^3 of air at STP would have a mass of about 18.22 grams.

1 ft^3 of water equals 28.32 Liters of water.
1 Liter of water has a mass of 1 kg, so 28.32 Liters of water would have a mass of 28.32 kilograms.

The net boyant force is the difference between the two weights...the mass of water displaced - mass of the displacing substance multiplied by gravity. However, if we instead think about the net "lifting power", if you will, and consider the mass which will be able to be supported by the net boyant force, we can ignore gravity (but remember why we are doing it).
28320 grams - 18.22 grams = 28301.78 grams.
28301.78 grams = 28.30 kg.

Now, going back and finding the net boyant FORCE, multiply by gravity (9.81 m/s^2),
28.30 kg * 9.81 m/s^2 = 277.6 Newtons

Thus, 1 ft^3 of air would be able to support an object weighing 277.6 Newtons.
The object to be supported would have a mass of 28.3 kg (62.3 pounds).

Answer 2

OK, this is a trick question because it depends on your requirements and what is being supported.

The straight out answer is 62.4 lbs per cubic foot.
That would be the case where you put a platform on the tubes and want to know how much the platform and its contents can weigh before it sinks down to water level.

Another way of looking at it is the case where the load that it is supporting is suspended from the tubes, like an anchor. Unless the object suspended from the tubes is a point mass, it will have volume and some buoyancy of its own. Even an anvil will weigh less underwater than it does on dry land, so the answer two the second part is "Depends on the nature of what you're supporting."

A couple of inner tubes will easily support a pair of 200 lb individuals in the water since their density is close to the density of the water.

Answer 3

How much have you got?
Say for example you have a ten foot tube that holds ten cubic feet. If you seal the first foot off with a piston or whatever the air cannot be compressed out of existence.
It would depend on the tensile strength of the tube at what point it would compress enough to blow out the side of the tube.
A plastic tube would blow faster than a steel tube. If nine cubic feet isn't enough weight make the imaginary tube 100 ft long.As long as the tube doesn't explode the cubic foot of air will hold it up.
The correct answer is infinite

Answer 4

1 ml of water equals 1 gram. 1 ml = 1 cm^3. There are 30 cm/foot, therefore 30*30*30 = 27000 ml in 1 cubic foot of water. Therefore 1 cubic foot of water weighs 27000 grams or roughly 58 pounds (480 grams per pound). Therefore if the container supports more than 58 pounds, it displaces only 58 pounds of water, therefore it sinks. If it supports less than 58 pounds, it will float.

Answer 5

If the container is like a life support jacket on water, it supports 63 lbs approximately. But what are the two tubes you talk about in your details to the question?

Answer 6

here is the calculations:

density of air at about 70 deg F at sea level is roughly 1 kg/m^3
density of fresh sea water is roughly 1000 kg/m^3

a cubic foot is .0283 cubic meters meaning it will hold up ~28 kg (~62 lbs)

Answer 7

62.4 pounds -- the weight of a cubic foot of water. Strictly speaking, you'd have to subract the weight of a cubic foot of air, but that's less than an ounce.

Answer 8

a cubic foot of water weighs (figures I gathered) 63.31lbs so a container of 1 cubic foot in size will displace 63.31lbs of water thus will hold up anything less than 63lbs of weight in water.

Answer 9

Water weighs about 8 pounds per gallon. I don't know how a gallon compares to as square foot but if you translate that volume to common terms, you should be able to determine how much water can be displaced and consequentally lifted.

Answer 10

Eureka, exactly the weight of one cubic foot of water.