A 8.00 L cylinder containing SF6 at a pressure of 0.400 atm is connected by a valve to 91.0L cylinder containing 02 at 342 torr pressure. Both cylinders are at 8.00 C. Calculate the partial pressure (torr) of SF6 when the valve is opened and the temperature of the cylinders changes to 78.0 C.

2 answers

Use PV = nRT to calculate mols SF6 in the 8.00 L cylinder and do the same with the 91.0 L cylinder to calculate mols oxygen. Then add to determine total mols of both gases, and use PV = nRT (remember to change T to 78.0 + 273) and determine pressure of the gases under the new volume (add the volumes, too). Don't forget to change 342 torr to atm. Post your work if you get stuck.
I slept on this problem over the night and realized early this morning that I didn't answer your question. There isn't anything wrong with what I wrote; I just didn't answer the question. You asked for the PARTIAL pressure of ONE of the gases; I gave you the TOTAL pressure of BOTH gases.
So you need not go through the above for both gases, just the one for which you want the partial pressure. Let me know if this isn't clear.