# Homework Help: Ideal Gas Law -- Isobaric Epansion followed by...

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1. Jun 8, 2017

### physics123

1. The problem statement, all variables and given/known data

An ideal gas with Cv = 5/2R, and γ = 1.4 starts at a volume of 1.5m3 , a pressure of 2.0×105Pa, and a temperature of 300K. It undergoes an isobaric expansion until the volume is V , then undergoes an adiabatic expansion until the volume is 6.0m3 , and finally undergoes an isothermal contraction until it reaches the original state.

2. Relevant equations
What is the volume V of the gas before the start of the adiabatic expansion?

3. The attempt at a solution
The answer is 2.23m^3 but I am unsure of how to go about getting it. I tried PV=nRT but I do not know 'n'.

2. Jun 8, 2017

### TSny

Hello.

You don't need to find the number of moles to work this problem even though there is enough information to determine n if you wish.

You are going to need more than PV = nRT to work the problem. There are basically three states involved in the problem. You know the types of processes that connect these states. Use this information to determine relations between P and V for these states.

3. Jun 8, 2017

### physics123

It seems that whichever way I try to solve this, I keep getting back to the fact that I don't know the temperature before the adiabatic change. That means I do not have the volume or the temperature at that point.

4. Jun 8, 2017

### TSny

Yes, the temperature and volume of the initial state of the adiabatic process are unknowns. However, there are relations that you can write down. For example, what do you know about adiabatic processes?

5. Jun 8, 2017

### physics123

adiabatic processes have no heat transfer (∆Q)

6. Jun 8, 2017

### TSny

What else do you know about adiabatic processes that might be helpful in determining P and V?

7. Jun 8, 2017

### physics123

I am completely lost beyond that point.

8. Jun 8, 2017

### TSny

You should review your notes or textbook regarding adiabatic processes for ideal gases and then come back to this problem.

9. Jun 8, 2017

### physics123

Don't you think I would've done that before posting in a forum? Thanks for your help.

10. Jun 9, 2017

### BvU

And did you find something involving adiabatic and gamma ?