- #1
Preston
- 3
- 0
I was assigned a problem in my Engineering Thermodynamics class as follows:
Problem:
An ideal gas in a rigid closed container undergoes isochoric heating from T1 = 27 C to T2 = 77 C. Initial gage pressure is 300 kPa, pressure of surroundings is 1 atm.
Find the final gage pressure.
The way to go about solving the problem seems straight forward, as I will show my train of thought below. The only thing I am stuck up on is what R value to use for this particular ideal gas, since the gas itself is not specified, nor it's volume, nor it's specific volume, nor it's mass.
Relevant Equations:
Pv = RT
Attempted Solution:
Here's how I attempted it:
Knowns: T1 = 27 C, T2 = 77 C, Pgage1 = 300 kPa, Psurround = 1 atm
Trying to find: Pgage2
For an ideal gas, Pv = RT, where v = specific volume (m3/kg) and R = specific gas constant (J/kgK). If I know what R value to use, I can solve for v:
v = (RT1)/P1 where P1 = Pgage1+Psurround (in Pa)
Then, because volume V is constant, and the unknown arbitrary mass m is constant, so is v
by the relationship
v = V/m
Then I would find P2:
P2 = (RT2)/v
It follows,
Pgage2 = P2 - Psurround (in Pa)
That's all folks. Maybe I am missing something rather obvious here, or taking the wrong approach. Again, the only thing I am stuck on is what the specific gas constant R should be. I can not solve for it with it's definition since R = R/M = nR where R is the universal gas constant (8.314 J/molK), M is molar mass of the gas, and n is moles of the gas. I have consulted Tables in the back of my text, and R is given for monatomic or diatomic ideal gas as is to be expected, but again, I'm not given those specifics in the problem. Any help is appreciated! :)
- Preston
Problem:
An ideal gas in a rigid closed container undergoes isochoric heating from T1 = 27 C to T2 = 77 C. Initial gage pressure is 300 kPa, pressure of surroundings is 1 atm.
Find the final gage pressure.
The way to go about solving the problem seems straight forward, as I will show my train of thought below. The only thing I am stuck up on is what R value to use for this particular ideal gas, since the gas itself is not specified, nor it's volume, nor it's specific volume, nor it's mass.
Relevant Equations:
Pv = RT
Attempted Solution:
Here's how I attempted it:
Knowns: T1 = 27 C, T2 = 77 C, Pgage1 = 300 kPa, Psurround = 1 atm
Trying to find: Pgage2
For an ideal gas, Pv = RT, where v = specific volume (m3/kg) and R = specific gas constant (J/kgK). If I know what R value to use, I can solve for v:
v = (RT1)/P1 where P1 = Pgage1+Psurround (in Pa)
Then, because volume V is constant, and the unknown arbitrary mass m is constant, so is v
by the relationship
v = V/m
Then I would find P2:
P2 = (RT2)/v
It follows,
Pgage2 = P2 - Psurround (in Pa)
That's all folks. Maybe I am missing something rather obvious here, or taking the wrong approach. Again, the only thing I am stuck on is what the specific gas constant R should be. I can not solve for it with it's definition since R = R/M = nR where R is the universal gas constant (8.314 J/molK), M is molar mass of the gas, and n is moles of the gas. I have consulted Tables in the back of my text, and R is given for monatomic or diatomic ideal gas as is to be expected, but again, I'm not given those specifics in the problem. Any help is appreciated! :)
- Preston