# Homework Help: Thermodynamics gases

1. May 11, 2016

### Mrencko

1. The problem statement, all variables and given/known data
two cubic ft, of air at 85 f and 13.9 psi, compress to 115 psi.
What is the final temp, the work, the internal energy "delta u", and Q

2. Relevant equations
well i dont have any equations, i am asking for the equations

3. The attempt at a solution
i dont know how to aproach to this problem because i dont know if this is
Isothermal
i am sufing the web, but there is a ton of equations out there and i feel overwhelmed about it, so i coul use some help to make a solid base to start in thermodynamics

Last edited: May 11, 2016
2. May 11, 2016

### Staff: Mentor

Isobaric means "at constant pressure" - can you compress something in an isobaric process?

3. May 11, 2016

### Mrencko

sorry i made a mistake it was
Isothermal

4. May 11, 2016

### Staff: Mentor

If it was isothermal - does it make sense to ask about the final temperature?

5. May 11, 2016

### Staff: Mentor

I think they meant for you to assume that the compression is adiabatic and reversible, although this certainly isn't clear from the problem statement. Have you learned the equation for the first law of thermodynamics? If so, why didn't you list that as one of your equations? What about the ideal gas law; did you not learn about that?

6. May 11, 2016

### Mrencko

Yes i know these equations, the ideal gas is pv=nrt and firts law of thermpfynamics its w= Q-internal energy

7. May 11, 2016

### Mrencko

you are right, it was so obvious, sorry its adiabatic because everything changes
how i should use this kind of info "one lb of air"? i dont see any weight or mass unit in equations, just "n" but if its air how i can know the moles of "air" i can if its two gases or one, for this problem i made the calculatins to find temperature final, and volume final, but i am stuck at the work calculation i have all exept the "n"

i convert all to non english units

V1=58.6337 lt
T1=302 k
P1=0.9458389 ATM
P2=7.8252 ATM
equations***
P1V1γ=P2V2γ
T1V1γ-1=T2V2γ-1

using that i found****
T2=552.34039 K
V2=12.9617 LT
now i just need the W and internal energy because the problem ask for Q but in adiabatic Q=0 so i need a litle litle help

i have this formulas for work***

W=-ncvλT→W=-ncv(T2-T1)
W=-(Cv)/(R)(P2V2-P1V1)
whic one its correct? and how i can know n?

Last edited: May 11, 2016
8. May 11, 2016

### Staff: Mentor

To get n, use PV=nRT together with the initial conditions.

9. May 11, 2016

### Staff: Mentor

Both equations are correct, and both should give the same answer.

10. May 11, 2016

### Mrencko

should i use R=(0.0820574)ltatm/kmol?
and what p and v, should work for p1v1=nRT1 and for p2v2=nRT2, i mean those two equations are valids or just whit initial valules, also if W=-internal energy any value of work its equal to a loss of internal energy and there is no need to calculate it_? also another question what if the problem says "1 lb of air" how i should consider the mass in equations?

Last edited: May 11, 2016
11. May 11, 2016

### Staff: Mentor

I'll leave it to you to work this out, considering the units that are needed.
Try both equations, and see what you get.
The first equation you gave actually is that change in internal energy.
I don't understand this question. Are you asking how to convert lbs to kg, grams, or gram moles?

12. May 11, 2016

### Mrencko

Well my other problem are the "same" exept for the mass thing, this is an example:
"one diesel machine have 159.09 grams of air, compress from 14.7 psi to 465 psi, whith 23.8 c of initial temp and 525.55 of final temp, calculate w and Q" its the same but the grams or lb, thing i dont know how to handle in the calculations

13. May 11, 2016

### Staff: Mentor

You don't know how to convert from grans to gram-moles, correct? The molecular weight of a material is equal to the number of grams per gram mole of material. So, to convert from grams to gram-moles, you divide by the molecular weight.

14. May 11, 2016

### Mrencko

thanks for the response, but i still have doubts, the "air its a mix" i get the idea of gramss/mols of x given gas or two, tree but what gases i should consider for air and proportions?

15. May 11, 2016

### Staff: Mentor

Use 21 mole percent for oxygen and 79 mole percent for nitrogen. This gives a molecular weight of (0.21)(32)+(0.79)(28)=29

16. May 11, 2016

### Mrencko

thank you very much i was really piss of about this because the teacher dint explain nothing at all, many thanks