Posted by Anna on Sunday, November 9, 2008 at 3:57pm.
Helium gas, He, at 22 degrees C and 1.00 atm occupied a vessel whose volume was 2.54 L. What volume would this gas occupy if it were cooled to liquidnitrogen temperature (197 degrees celcius).
What is the equation?

AP Chem  Trig, Sunday, November 9, 2008 at 4:03pm
Always start with PV/nT = PV/nT, where P is the pressure, V is the volume, n is the number of moles, and T is the temperature. Take out whichever terms you don't need. In this case, you don't need n (the number of moles), so you're left with:
PV/T = PV/T
Solve for the V on the right side of the equation after you plug in everything else. 
AP Chem  Anna, Sunday, November 9, 2008 at 4:12pm
I did that and got 22.7.
1st of all the answer is supposed to be .65 L
2nd of all where does the negative go? 
AP Chem  Trig, Sunday, November 9, 2008 at 4:09pm
( You only use PV=nRT when none of the variables change. In this case the temperature is changing, so you use PV/nT = PV/nT )

AP Chem  Trig, Sunday, November 9, 2008 at 4:25pm
Temperature must always be in Kelvin! Add 273 to the temperature in Celsius to get the temperature in Kelvin. In this case:
INITIAL TEMPERATURE = 22+273 = 295 K
FINAL TEMPERATURE = 197+273 = 76 K 
AP Chem  Anna, Sunday, November 9, 2008 at 4:28pm
ooh ok that explains it lol thanks =]