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[clarence]

I came across this thermodynamics problem in my course of engin thermo:

"A spherical balloon (that is connected to valve that is initially closed) initially contains 25m3 of helium gas at 20C and 150kPa. A valve is now opened and the helium is escape slowly. The valve is closed when the balloon drops to the atmoshperic pressure of 100kPa. The elasticity of the balloon material is such that the pressure in the balloon varies with the volume according to the relation: P = a + bV, where a = -100kPa, and b is a constant.

Ignoring any heat transfer, and assuming constant specific heats for helium, find (a) the final temperature in the balloon, and ( the mass of the helium that escaped."

Cp(He) = 5.1926 kJ/kg/K
Cv(He) = 3.1156 kJ/kg/K

You may use teh uniform flow assumption.

Appreciate any help from anyone.

cheers

[gvdlans]

At the start of your experiment you know:
- pressure
- temperature
- volume
- composition (= pure helium)

Hence you can calculate the initial mass via the ideal gas law.

At the end of your experiment you know:
- pressure
- composition (still pure helium)

You can calculate:
- volume (based on the given P-V relationship)
- temperature (based on equation for isenthalpic expansion and the given Cp and Cv values)

and with these results you can calculate the final mass with the ideal gas law. Difference between the initial mass and the final mass is the mass escaped.