# Calculating Air Pressure

1. Jun 12, 2013

### aaronfue

1. The problem statement, all variables and given/known data

I'm trying to calculate the air pressure from some data that I have.

#1. Calculate the cylinder pressure. (barometric pressure=765 mmHg, h=35mm)

#2. Calculate the moles of air using low temperature and volume and cylinder pressure above. (Low temp=50.0deg Celsius, vol=4.0mL, R=62.36)

#3. Calculate the air pressure using the high temperature reading from data. (High temp = 80deg Celsius)

2. Relevant equations

Ideal Gas Law
PV=nRT

3. The attempt at a solution

I have converted the temps to Kelvin and the mL to L. I have also made a -0.2 mL correction to my volume.

Question #1= 768 mm Hg

Question #2 = 1.448×10-4 mol air

Question #3 = The question doesn't say to use the volume for that high temp, so I use it anyways. Another classmate got 420 mm Hg. But I don't get that. I get 480+ mm Hg.

Should I use the volume from question #2 since #3 doesn't specify?

The data that was collected is the same as my classmates but I don't know where I am making my mistake!!

I would appreciate any assistance with this!!

Last edited: Jun 13, 2013
2. Jun 13, 2013

### Staff: Mentor

Cylinder? h? You have to explain what you are talking about.

3. Jun 13, 2013

### aaronfue

I'm eventually going to find the enthalpy of vaporization of water. The pressure of the cylinder is what needs to be determined which is asked in the first question.

The value of h is given in the first question in parenthesis. I was able to calculate the cylinder pressure. I just needed help with my moles of air from the second question.

4. Jun 13, 2013

### Staff: Mentor

You have still not explained what the h is. I can do some guessing, but that would be a waste of time, as I don't know what is the setup, if the cylinder is rigid, or closed with a piston, why the pressure inside is different from atmospheric and so on. Basically I know nothing.