The vapor pressure, P, of a certain liquid was measured at two temperatures, T.

Question

The vapor pressure, P, of a certain liquid was measured at two temperatures, T. The data is shown in this table. If you were going to graphically determine the enthalpy of vaporization, Delta H_vap, for this liquid, what points would you plot? Point1: avoid rounding errors, use three significant figures in the x values and four significant figures in the y values. point 2: Determine the rise, run, and slope of the line formed by these points. What is the enthalpy of vaporization of this liquid? Delta H_vap =

Explanation / Answer

We need to plot lnP on y axis and 1/T on x axis
P = 3.85
ln P = ln (3.85) = 1.348
T = 275 K
1/T = 1/275 = 0.003636

point 1 is:
x = 0.003636
y = 1.348

P = 8.48
ln P = ln (8.48) = 2.138
T = 525 K
1/T = 1/525 = 0.001905

point 2 is:
x = 0.001905
y = 2.138
----------------------------------------------------------------------------
rise = (y1-y2) = (1.348 - 2.138) = -0.79
run = (x1-x2) = (0.003636 - 0.001905) = 0.001731

slope = rise/run
= -0.79/0.001731
= - 456.38
-----------------------------------------------------------------------------
use:
slope = - delta Ho/R
-456.38 = - delta Ho / 8.314
delta Ho = 3794 J/mol

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