# AP Chemistry

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These are some lab questions from the following lab: Vapor Pressure and Enthalpy of Vaporization of Water

1. Why does vapor pressure change with temperature?

2. The assumption was made that the vapor pressure of water is negligible at a temperature close to zero. Find the actual vapor pressure of water at you low temperature and comment on the validity of the assumption.
(what equations/calculations do I use?)

3. The assumption was also made that the slight changes in "h", the depth under the surface of the water, will not significantly change the total pressure in the graduation cylinder. Comment on the validity of this assumption.

4. Write out the long "two-point" form of the Clausius-Clapeyron equation. Why does the graphical of analysis give a better value for the enthalpy of vaportization than does the form of the Clausius-Clapeyron equation using two temperature-vapor pressure values?

1. Why does vapor pressure change with temperature?
Think kinetic molecular theory. Look up the KMT and explain from there.

2. The assumption was made that the vapor pressure of water is negligible at a temperature close to zero. Find the actual vapor pressure of water at you low temperature and comment on the validity of the assumption.
(what equations/calculations do I use?)
I'm not sure what you did; however, I think the question is asking you to measure the vapor pressure at the lowest temperature at which the experiment was conducted. That probably means you have the number somewhere in the data you collected. If that isn't correct, then I would interpret it to mean that you look up the value of the vapor pressure in a set of tables.

3. The assumption was also made that the slight changes in "h", the depth under the surface of the water, will not significantly change the total pressure in the graduation cylinder. Comment on the validity of this assumption. You can calculate the pressure of a column of water h cm high and that will allow you to compare with the pressures you used.

4. Write out the long "two-point" form of the Clausius-Clapeyron equation. Why does the graphical of analysis give a better value for the enthalpy of vaportization than does the form of the Clausius-Clapeyron equation using two temperature-vapor pressure values?
My take on this is that the graphical solution provides a smoothing out effect over many points. With a two-point solution, you have a straight line, period.

thankyou !!

hey--we just did this lab! and ahhh! i'm still clueless about the two point clausius-clapeyron eq. anyone have any more help?

We just did this same lab also and it was so confusing, but you helped just a little bit hee hee internet is great!!! :)

• chemistry -

what are the 2 assumptions of KMT that maybe considered vaque

• AP Chemistry -

Two- point equation:
ln (P1/P-2) = - (ÄHvap/R)(1/T2-1/T1)

The graphical analysis gives a more exact value because it takes into account possibly erroneous data points, making an “average”, “trend” or “best fit” line that is as close as possible to all data points. The equation method only utilizes two of the data points, which may or may not be representative of the rest of the data, and which may or may not be skewed.

• AP Chemistry -

thank you!! i was stuck on the last question but this helped :))

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