# Homework Help: Pressure Question

1. Jan 31, 2005

### Frustra8ed

I know this question is easy, but for some reason I am unable to figure it out.

The first part of the problem wants to know what the volume is in a pressurized tank. I figured it and it equals 2.1 ft^3.

The second part asks for the specific volume at 70 F and 200 psi.
So far all I have is v(specific volume) = V/m. Or 1/d = v

The gas in the tank is air, so basically I need to find the mass of air @ 70 F and 200 psi. I have the volume, so that is not the problem.

The answer to the question is .98 ft^3/lbm

Should I be using ideal gas laws? If I do use the ideal, I'm assuming I need to convert units. I'm having a problem converting psi to lbf/ft^2...

I can't come up with this answer, so if anyone can give me a hand I would appreciate it.

Thanks,

Frustr8ed, as the name states.

2. Jan 31, 2005

### Andrew Mason

It would help if you would state the whole question. Use the ideal gas equation of state, PV=nRT to find n (number of moles). The answer won't be in ft^3/lbm.

AM

3. Feb 1, 2005

### FredGarvin

You may want to consider using this form of the ideal gas law:

$$P \nu = RT$$

where:

$$P = Pressure (Lb_{f}/ft^2)$$
$$\nu = Specific Volume (ft^3/Lb_{m})$$
$$R = Universal Gas Constant (for air: 53.35 Ft*Lb_{f}/°R*LB_{m})$$
$$T = Temperature (°R)$$

4. Feb 1, 2005

### Frustra8ed

Thanks for the responses. I believe that last equation is the one I need to use, but I was having trouble converting units.

Thanks Again,

Frustra8ed