Calculating Pressure and Mass of Ethane Gas in a Flask

[janiexo]

I'm really struggling with this concept and can't seem to get my head around it. I don't know why because it seems simple enough but I can't seem to get the answer :

Question:
A flask with a volume 'V', provided with a stopcock, contains ethane gas (C2H6) 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.

a) What is the final pressure of ethane in the flask?
b) How many grams of ethane remain in the flask (use 'R' for ideal gas constant)?

 

[HallsofIvy]

Okay, first what is the "ideal gas equation"? What are the starting pressure, temperature and what are the pressure and temperature just before the stopcock is closed? What must stay constant once the stopcock is closed? So what must change? What are the temperature and pressure at the end?

 

[janiexo]

Ok, well the equation is pV = nRT
So at the start it's pressure is p_0 and temperature T_0
Before it's closed the temperature is T and the pressure is ... I don't know what the pressure is, would it stay at p_0?
So once it's closed the volume stays constant so the pressure must change. The temperature at the end is T_0 once again but i have no idea about the pressure.

 

[quark]

HallsofIvy gave you good pointers. During each process step, there is one property that remains constant. Try to understand what Charles and Boyle's laws say(in the same order).

 

[~angel~]

Ok, well the equation is pV = nRT
So at the start it's pressure is p_0 and temperature T_0
Before it's closed the temperature is T and the pressure is ... I don't know what the pressure is, would it stay at p_0?
So once it's closed the volume stays constant so the pressure must change. The temperature at the end is T_0 once again but i have no idea about the pressure.

Use (p_1*V_1)/T_1 = (p_2*V_2)/T_2

You know volume remains constant, you know the temperature, then you can work out the final pressure.

Then to find the mass, just use pV = nRT.

 

[janiexo]

Thanks for your help everyone, I understand it now. Things seem a lot clearer in the light of day I guess :)