# Homework Help: Gas expansion

1. May 9, 2005

### WY

Hey - I'm doing this question (see below) but I dont hav a clue where to start!!!

A flask with a volume of V, provided with a stopcock, contains ethane gas at a temperature of T_0 and atmospheric pressure p_0. The molar mass of ethane is M. The system is warmed to a temperature of T, with the stopcock open to the atmosphere. The stopcock is then closed, and the flask cooled to its original temperature.

What is the final pressure of the ethane in the flask?

and

How many grams of ethane remain in the flask? Use R for the ideal gas constant.

for the first part i tried relating the initial and final pressures and volumes as a ratio... but that didn't work and since I dont have the volume of the actual gas which changes I cant apply the pV=nCRT equation...
Can someone tell me how to start?

2. May 9, 2005

### OlderDan

I don't know about the C in pV=nCRT. The usual representation of the ideal gas law is PV = nRT, where R is a constant. In this problem, the pressure is constant during heating, and the volume of the flask is constant for the whole problem. During heating, PV is constant and you are raising the temperature. What has to change, and how is that change related to the temperature change?

3. May 10, 2005

### WY

Thanks for replying! Sorry I wrote down the wrong eqution -_-
but in this problem doesnt the volume of gas change? So why doesnt it effect the way we caluclate it?

4. May 10, 2005

### whozum

Nope, the problem essentially starts when the cork is closed, after which the volume is constant. I dont know why they included all that information, its just confusing, they should have just stated that the system started at a certain temperature and dropped.

5. May 10, 2005

### WY

oh I see!! thanks for clearing that up now - its all crystal clear now!!!