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Topic: Thermodynamics - First Law Problem  (Read 1907 times)

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Offline YuliaSnow

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Thermodynamics - First Law Problem
« on: September 25, 2016, 07:08:45 PM »
I have been trying to figure out how to do this problem for a few hours, hoping someone can help me out. :)


An air mattress (of volume 50L) is inflated using a solid called  "A" with a molar mass of 10g/mol. "A" decomposes and produces "B", a gas at a ratio of 1:1.5. Each mole of A also produces an enthalpy of ΔH=-10kJ. Assume 70% of this energy is deposited in "B" and decomposition is instantaneous. Also let "B" be an ideal gas with Cp,m=30J/mol/K. In order for the mattress to work properly, it should reach a pressure of 3 atm when fully inflated.

a) Estimate the mass in g of "A" needed to use the air mattress. State any assumptions or approximations being made.
Hint: Treat the decomposition of "A" and the expansion of "B" as two separate processes.

b) Find the temperature of "B" after the mattress is fully inflated.
Here is my attempt at it:

I assumed the temperature is at room temp (298K) and pressure is starting at 1 atm.

I used q=(10kJ/mol*1000J/kJ*30%*2/3*n) since 70% was put in "B".
Some equations I used:
dU=CvdT for ideal gas




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