Volume, pressure and final temperature

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SUMMARY

The discussion focuses on a thermodynamics problem involving a mass of air with an initial temperature of 500°C and pressure of 0.8 MPa, expanded to a final volume of 90 liters. The key equations utilized include the relation pV1.2 = c and the ideal gas law pV = nRT, where R is the specific gas constant for air (287 J/kg·K). Participants calculated the number of moles of air (n) as approximately 4.14 and discussed how to derive the initial volume and final pressure and temperature based on the provided conditions.

PREREQUISITES
  • Understanding of the ideal gas law (pV = nRT)
  • Familiarity with specific gas constants (R = 287 J/kg·K for air)
  • Knowledge of thermodynamic principles, particularly the relation pV1.2 = c
  • Basic conversion skills between Celsius and Kelvin
NEXT STEPS
  • Calculate the initial volume (Vi) using the ideal gas law with the known values.
  • Determine the final pressure (pf) after expansion using the relation pV1.2 = c.
  • Find the final temperature (Tf) using the ideal gas law after determining the final pressure.
  • Explore the implications of the polytropic process represented by pV1.2 = c in thermodynamic systems.
USEFUL FOR

Students studying thermodynamics, engineers working with gas laws, and anyone involved in HVAC or fluid dynamics applications.

Ben_Walker1978
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Homework Statement



a) A mass of 0.12 kg of air has an initial temperature of 500°C and pressure 0.8 MPa. If the air is expanded according to the law pV1.2 = c to a final volume of 90 litres, determine
i) its initial volume,

ii) its final pressure,

iii) its final temperature.
For air, take R = 287 Jkg¯¹ K¯¹.

Homework Equations


pV1.2 = c
pV = nRT
n = pxV / RxT

The Attempt at a Solution



Can anyone point me in the right direction?
As i have no clue how to complete this.
Thanks.
 
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We aren't allowed to give the answer unless you show us your work attempting a solution. "No clue" is not enough.
 
Hello.

i have done a bit of working out.

I converted celsius to kelvin. Which is 773.15

i think 1.2 is how dry the air is?

n = 0.8 x 90 / 287 x 773.15 = 193.96

So n = 193.96

So i have all the information for the equations.

Is is just pV1.2 = c
 
Ben_Walker1978 said:
n = 0.8 x 90 / 287 x 773.15 = 193.96
This will not work. You have mixed the initial pressure and final volume.

Part one asks for the initial volume. How could you use the ideal gas law and the given relation ##1.2pV=c## to find this?
 
Last edited:
How many gram moles of air does 0.12 kg represent?
 
Last edited:
i thought i worked this out in my previous post.

The molar mass of air is 29g/mol
 
Ben_Walker1978 said:
The molar mass of air is 29g/mol
So what is ##n## in this case?
 
Ben_Walker1978 said:
i thought i worked this out in my previous post.

The molar mass of air is 29g/mol
I don't see it there. If the molar mass of air is 29 g/mol, how many gram moles of air does 0.12 kg represent? It is definitely not 194.
 
3.48?
 
  • #10
Ben_Walker1978 said:
3.48?
Please show how you arrived at this number?
 
  • #11
I have worked it out i think.

0.12kg / 0.029kg/mol = 4.1379

Is this correct?
 
  • #12
Ben_Walker1978 said:
I have worked it out i think.

0.12kg / 0.029kg/mol = 4.1379

Is this correct?
Yes. Now you know the number of moles, the initial temperature, and the initial pressure. So, from the ideal gas law, what is the initial volume ##V_i##?
 

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