Calculating Pressure in a Meteorological Balloon at -20.0C

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To calculate the pressure inside a meteorological balloon at -20.0°C, the ideal gas law is applied using the formula PV = nRT. The balloon's initial volume at sea level is 4.187 m³ with an assumed initial pressure of 1 atm and temperature of 273K. At maximum altitude, the balloon's volume expands to 113.04 m³ and the temperature drops to 253K. Using the equation P1V1/T1 = P2V2/T2, the calculated pressure at altitude is approximately 0.03 atm. The assumptions made regarding standard temperature and pressure (STP) are considered reasonable for this calculation.
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Homework Statement

A meteorological baloon had a radius of 1 m when released from sea level and expanded to a radius of 3 m when it had risen to it's maximum altitude, where the temperature was -20.0C. What is the pressure inside the balloon at that altitude?



Homework Equations

PV = nRT (I assumed ideal behavior)
n & R are constant, so
P1V1/T1 = P2V2/T2



The Attempt at a Solution


known:
initial volume= 4/3pi (1m)^3=4.187 m^3
final temp = 253K
final volume = 4/3 pi (3)^3=113.04 m^3

Can I assume STP at sea level and use 1 atm for initial pressure and 273K for initial temperature? Or is there more to it than that?

If I do that I get .03 atm. This sounds reasonable, but I am not sure the assumtions I made are correct.
 
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Your assumptions sound pretty reasonable to me.
 
The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?

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