# Need help calculating number of moles

1. Jun 22, 2017

### Jordan Michaels

1. The problem statement, all variables and given/known data
A bubble rises from the bottom of a lake of depth 80.0 m, where the temperature is 4oC. The water temperature at the surface is 18oC. Assume the bubble to be in spherical shape and has an initial diameter of 1.00 mm. Assume that the pressure at the surface of the lake is 1.0 atm. Calculate a) the pressure at the bottom b) the number of moles and molecules in the bubble and c) the diameter of the bubble at the surface.

2. Relevant equations
PV=nRT
P(bottom) = P(atm) + d(ro)*g*h

3. The attempt at a solution
I have solved part a and c but I cannot answer part b. The answer to part a is 885300 Pascals and the answer to part c is 2.09 mm but no mater what I try I cannot get the correct answer to past b.

2. Jun 22, 2017

### Buzz Bloom

Hi Jordan
Sorry - I answered the wrong question, so I erased it.

Problem b: I think you need to make an assumption about what the stuff in the bubble is. Do you have a guess? Also, you will need to use the Ideal Gas Law. Do you know what that is?

Regards,
Buzz

Last edited: Jun 22, 2017
3. Jun 23, 2017

### Trollfaz

using
PV=nRT
did you get 2.01*10^-7mol
Note that the gas in the bubble is deviating significantly from an ideal gas under such high pressure and low temperature

4. Jun 23, 2017

### haruspex

Are you sure?

5. Jun 23, 2017

### Staff: Mentor

Air at about 9 bars and 4 C does not deviate much from ideal gas behavior. What is your estimate of the compressibility factor Z at these conditions? I guess it all boils down to what one considers "significantly."

6. Jun 23, 2017

### Buzz Bloom

Hi haruspex:

Thanks for the question. My mistake was thinking about determining the mass of the bubble rather than the moles.

Regards,
Buzz