AP Chem
posted by Anna .
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?

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. 
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? 
( 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 )

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 
ooh ok that explains it lol thanks =]