Homework Help: Volume calculation

1. Jun 13, 2007

eldon

how do I calculate the volume of air in a sphere if I know

2. Jun 13, 2007

cepheid

Staff Emeritus
When you say in a sphere, do you mean "in a spherical container?" This is the only thing that makes sense to me. If so, then the gas will fill the container, and all you need to know is the volume of the sphere. For that, all you need to know is the radius.

3. Jun 13, 2007

eldon

its not a rigid tank it is rubber and can stretch

4. Jun 13, 2007

cepheid

Staff Emeritus
In that case, couldn't you just use the ideal gas law?

5. Jun 14, 2007

eldon

yea but how does the radius figure in

6. Jun 14, 2007

eldon

air volume in a balloon

How do I figure out the volume of air in a balloon if I know the radius of the balloon, the air temp, and the pressure?

7. Jun 14, 2007

Staff: Mentor

If it's a sphere, all you need is the radius to compute the volume.

8. Jun 14, 2007

haiha

You know the geometric volume of the balloon. You know the pressure, the temp. Apply the real gas law formula:

P1V1/T1 = PoVo/To

9. Jun 14, 2007

eldon

so the volume of the sphere using 4/3*pi*r^3 would be the same as the volume of the air?

10. Jun 14, 2007

eldon

how does that relate to the radius of the balloon?

11. Jun 14, 2007

Staff: Mentor

Sure. Simple as that.

12. Jun 14, 2007

cepheid

Staff Emeritus
Maybe this is clear by now, based on other people's responses, but if you know P and T, then the radius is predetermined. It is not an independent variable. The reason is that for an ideal gas, P and T uniquely determine V, which in turn contrains the radius, since the volume is a sphere.