# Ideal gas question on oxygen

1. May 7, 2006

### N_L_

A standard cylinder of oxygen used in a hospital has the following characteristics at room temperature (295 K): gauge pressure = 13,800 kPa (2000 psi), volume = 16 L (0.016 m^3). How long will the cylinder last if the flow rate, measured at atmospheric pressure, is constant at 2.4 liters/min?

I tried to find the volume (in liters) at atmospheric pressure. It didn't come out right.

Since nRT is constant do I need to find the number of moles?

Am I taking the wrong approach?

2. May 7, 2006

### Hootenanny

Staff Emeritus
Assuming temperature is constant you could use Boyle's law.

~H

3. May 7, 2006

### N_L_

Does that work for gauge pressure?

Thank you.

4. May 7, 2006

### Hootenanny

Staff Emeritus
You must first convert it to absolute pressure, remember;

$$P_{gauge} = P_{abs} - P_{atm}$$

~H

5. May 7, 2006

### LeonhardEuler

Just add atmospheric pressure to the guage pressure to get absolute pressure.

6. May 11, 2006

### N_L_

P absolute = P gauge + P atmospheric

= 13,800 kPa + 101 kPa

= 13,901 kPa

PV = k

If I don't need to know k, how to I use the equation?

7. May 11, 2006

### Hootenanny

Staff Emeritus
If you know the pV is constant you can make an equality with respect to the intial and final pressures and volumes, thus;

$$p_{i}V_{i} = p_{f}V_{f}$$

~H

8. May 11, 2006

### N_L_

Many thanks for all of the help.