# AP Chem

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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 -

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 -

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 -

( 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 -

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 -

ooh ok that explains it lol thanks =]

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