Thermodynamics - Find initial pressure (given Temperatures)

1. Sep 25, 2010

physics_197

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

For a certain experiment, R-410a vapor is contained in a sealed glass tube at 20°C. It is
desired to know the pressure at this condition, but there is no means of measuring it, since
the tube is sealed. However, if the tube is cooled to −20°C small droplets of liquid are
observed on the glass walls. What is the initial pressure?

2. Relevant equations

Thermodynamic tables
ideal gas equation?

3. The attempt at a solution

I set up both states:
State 2: T = -20 (saturated)

State 1: T = 20, v1=v2=0.0648 P = ? (superheated)

I just don't understand how we can figure it out from just the temps. I just cant wrap my mind around the thermodynamic tables.

2. Sep 26, 2010

Staff: Mentor

What happens at -20?

3. Sep 26, 2010

physics_197

At -20, the condensation occurs?

4. Sep 26, 2010

Staff: Mentor

Can you use it to find vapor pressure at -20? With the help of tables?

Last edited by a moderator: Aug 13, 2013
5. Sep 26, 2010

physics_197

Is it read directly from the table? would it be 399.6 kPa?

6. Sep 26, 2010

Staff: Mentor

No idea, I don't have these tables. But yes, you should read it directly from the table.

Last edited by a moderator: Aug 13, 2013
7. Sep 26, 2010

physics_197

So, how would I use this information to find the initial pressure?

Could I just use the ideal gas? P1/T1 = P2/T2 ?