Getting strange answer , lease help

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Discussion Overview

The discussion revolves around a thermodynamics problem involving the calculation of the mass of air in a specified process cycle, including isothermal compression, isobaric expansion, and isochoric cooling. Participants explore the application of the ideal gas law and the appropriate values for the gas constant.

Discussion Character

  • Homework-related
  • Technical explanation

Main Points Raised

  • One participant presents a problem involving the calculation of mass using the ideal gas law, initially using an incorrect value for the gas constant.
  • Another participant corrects the value of the gas constant, stating that it is not 6, which prompts a humorous reaction.
  • A later reply clarifies that the participant was using two different gas constants, identifying Rv as 6 and providing the correct value for the universal gas constant as 0.287 kJ/kg K.
  • The participant then recalculates the mass of air, arriving at a value of 29.90 g and seeks confirmation on the correctness of this result.

Areas of Agreement / Disagreement

There is no consensus on the final mass calculation, as participants have not explicitly agreed on the correctness of the final answer provided.

Contextual Notes

The discussion includes potential confusion regarding the appropriate gas constant to use, which may depend on the specific conditions and definitions applied in the problem.

xto
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The question is
a volume of 2.2m^3 of air at inital conditions of 1.2bar and 303K undergoes the following process to complete a cycle :

i)isothermal compresssion from state 1 through a volumetric compression ration of 6 to state 2

ii) isobaric expansion from state 2 back to the initial volume thus reaching state 3

iii) isochoric cooling from state 3 back to the original pressure to reach state 1

Q1) Find the mass of air present

so i use the formula PV=nRT

P=1.2bar
V=2.2M^3
n=?
R=6
T=303K

so i need to find N so i got 2.64 = 1818N
which N is 1.452x10^-3 :cry:
which is total not the mass of the air , please help
 
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R is the universal gas constant. It is not 6.
 
FredGarvin said:
R is the universal gas constant. It is not 6.
OMG, I just spit water all over my keyboard. :smile:
 
ok , i kind of get it
beacuse I'm given 2 R
Rv is 6
the other R is 0.287kJ/kg K

so its

(1.1832)x(2.2x10^3)
---------------------
(0.287)x(303k)

then i get 29.90g (to 2d.p) Air

is that right for the mass of air present? By the way , thank you so much for the help
 

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