Finding temperature through a manometer

AI Thread Summary
The discussion centers on using a mercury manometer to determine temperature changes in a gas cell under varying conditions. Initially, the mercury height is 120 mm at 0°C in an ice-water mixture, dropping to 30 mm in an industrial freezer. The user initially assumes constant pressure but later realizes that pressure changes while volume remains constant. The relevant equation for the problem is p1*v1/t1 = p2*v2/t2, but the user struggles to apply it correctly. Clarification on how to incorporate pressure changes into the calculations is needed for a proper solution.
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Homework Statement


The mercury manometer shown in the figure is attached to a gas cell. The mercury height h is 120 mm when the cell is placed in an ice-water mixture. The mercury height drops to 30 mm when the device is carried into an industrial freezer. HINT: The right tube of the manometer is much narrower than the left tube. What reasonable assumption can you make about the gas volume?
[PLAIN]http://img36.imageshack.us/img36/6331/knightfigure1650.jpg
I'm assuming the temperature is originally at 0C since it says it's in an ice-water mixture. I'm also assuming pressure is constant throughout the problem.

Homework Equations


V/T=V/T
C+273=K

The Attempt at a Solution


I attempted to do:
120/273K = 90/T
T=-68.25C
Wrong answer. I'm guessing there's more to put into the volume part of the equation than just the height, but I don't know exactly what. Can anyone help? Thanks.
 
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After much observation, I was wrong to conclude pressure would be the same. After reading the hint carefully, I came to the conclusion pressure changes, while volume remains constant. Still, I don't know how to find the pressure of the system.
 
p1*v1/t1=p2*v2/t2
 
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