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. If you were going to graphically determine the enthalpy of vaporizaton, AHap, for this liquid, what points would you plot? The data is shown in this table Number Number T (K) P(kPa) 325 3.53 725 8.13 To avoid rounding errors, use three significant figures in point 1 the x values and four Number Number NOTE: Keep the pressure units in kPa. significant figures in the y vaiues. point 2: Determine the rise, run, and slope of the line formed by these points. rise run slope Number Number Number What is the enthalpy of vaporization of this liquid? Number J/mol vap

Explanation / Answer

from classius -clayperon equation

ln(P2/P1)= (deltaH/R)(1/T1-1/T2)

where P2 is vapor pressure at T2( in K) and P1= vapor pressure at T1(inK)

so a plot of lnPvs 1/T gives a slope of -deltaH/R

hence y axis is vapor pressure and x axis is temperature in K

point -1 : x- axis is T1= 325K 1/T1= 1/325=0.0031, y axis is , lnP1= ln(3.53)= 1.26

point-2: x-axis is T2= 725K, 1/T2= 1/725= 0.0015, y axis is lnP2= ln(8.13)= 2.1

rise is difference in y axis = 2.1-1.26= 0.84 and run is difference in x axis =-0.0031+0.0015=-0.0016

slope = rise/run= -0.84/0.0016=-deltaH/R=-525

deltaH= 525R, R= 8.314 J/mole.K

deltaH= 525*8=4200J/mole.K=enthalpy of vaporization

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