## Ideal gas problems

There are some problems which have me completely stumped.

1. A balloon has a volume of 10.5dm^3. Its mass is 8.5g and inside is helium at a pressure of 1.05atm. Atmospheric pressure is 1.00atm and both the helium and the outside air is 25.0C (degrees Celcius). Define the tension in the balloons string. Molar mass of helium is 4.00g/mol and air's is 29.1g/mol.

I guess the tension is caused by the buouancy of the ball and can be expressed as T=B-w.

Now... I could solve this the traditional way by using the density of air, but the value is not given so there must be a way to get through this by using those molar masses. I have no idea how. Please help.

2. In another problem a diver has a tank of pressurized air. I am given the initial temperature, pressure and mass of the air as well as the values after the dive, the molar mass of air and the atmospheric pressure. I need to find out the volume of the gas. The air is considered to be a diatomic ideal gas.

Now I have the equation pV = nRT = (m/M)RT so I could directly solve for V with it being the only unknown. Now I wouldn't need the other set of values at all so this can't be right. The fact that the air is diatomic must chance something. I understand that there could now be collisions between the gas' atoms, but what effect does it have on this?

3. How do I calculate the average translation energy of non-ideal gas? I have 5.00l of hydrogen at atmospheric pressure and at a temperature of 25C.

For ideal gas I found the equation ½mv^2 = (3/2)kT where k is the Boltzman constant. There was no mention of the gas being ideal in the problem so I'm assuming it isn't. Any advice?

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