Homework Help: Calculate density of air from experiment.

1. Nov 5, 2012

spsa

1. The problem statement, all variables and given/known data
A balloon filled with 0,78 liters of air is placed in liquid nitrogen. After a while the ball is taken out of the liquid nitrogen and weighed immediately, the mass is 2,27 grams. After the ball has grown to its regular size it weighs 1,26 grams.
What value does the experiment give as the density of air?

3. The attempt at a solution

2,27-1,26= 1,01 grams (weight of contents)

1,01 grams = 0,00101 kg
0,78 L = 0,00078 m3

Density = mass/volume = 0,00101/0,00078 = 1,29 kg/m3

Will be very thankful for any/all help!

Last edited: Nov 5, 2012
2. Nov 5, 2012

collinsmark

Hello spsa,

Welcome to Physics Forums!
'Looks good to me!

(There are some approximations being made here, such as when the balloon has grown to its regular size, the temperature and pressure are the same inside and out. Although that's not absolutely true if the balloon maintains its elasticity, it should still give a very reasonable approximation.)

3. Nov 6, 2012

Basic_Physics

I think this method uses the buoyancy force to determine the density of air. It is assumed that the volume of the balloon is very small when it comes out of the nitrogen so that there is virtually no buoyancy. The difference in weight is then due to the bouyancy force it experiences once it is back to its original volume. So this method actually measures the mass of the air displaced by the volume of the balloon.