Homework Help: AP Chem

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 liquid-nitrogen 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 =]

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