# Vapor Pressure Help

1. Apr 10, 2009

### StephenDoty

Hello all

I completed a lab last week where we filled a graduated cylinder up 90% of the way with water and inverted it into a water bath and then we heated the bath to 80 degrees Celsius then measured the volume of the air bubble as it cooled to 50 degrees Celsius and at 1 degree Celsius.

My questions are given vapor pressure at 39.5 degrees Celsius is 400 torr. at 7.7 degrees Celsius the vapor pressure is 100 torr. What is the value for $$\Delta$$Hvap?

Ln(P2) - Ln(P1) = -$$\Delta$$H/R ( !/T2 - 1/T1)

$$\Delta$$Hvap= 3.18 * 10^4 J/mol
right??

And three assumptions made in obtain results in this lab would be:
1. the bubble of air follows the ideal gas laws and not other equations like the van deer walls equation.
2. there isn't any other pressures besides the atmosphere pressure, the water vapor pressure, and the pressure of the air. or that the atmosphere pressure stays the same.
3. that the air is homogeneous and will fill the volume evenly with an exact number of particles, mols, in the bubble.

Are these three assumptions ok?

Thanks for the help everyone.
Stephen

2. Apr 11, 2009

### Staff: Mentor

Can't think of better ones.

3. Apr 11, 2009

### StephenDoty

great! thanks.
but is Delta Hvap= 3.18 * 10^4 J/mol
right??