What pressure is necessary to change the boiling point of waterfrom its 1.000 at

Question

What pressure is necessary to change the boiling point of waterfrom its 1.000 atm value of 100oC to 97oC?The heat of vaporization of water is 40.7 kJ/mol. The density ofliquid water at 100oC is 0.958 g/mL and the density ofsteam is 0.5983 g/L

I figured you would use the following equation and solve forP2 but I am not sure how to incorporate density in thisproblem. Every other element is there I think or do I noteven need the density in this problem?

ln(p1/p2) = (Hvap/ R) (1/T2 - 1/T1)

I have looked in my P chem book and do not have any reference tousing density and I even sought the help of a friend who took thecourse several years ago. They had an older book, all oftheir notes and I am still can't seem to figure this one out. Any help is appreciated.

Explanation / Answer

P1 = 1.000 atm P2 = ? T1 = 100 C T2 = 97 C H vap = 40.7 kJ/mol R = 8.314 J/k. mol ln(p1/p2) = (Hvap/ R) (1/T2 - 1/T1) ln (1 / P2) = (40.7 / 8.314) ( 1 / 97 - 1/100)                = 0.00151       1 / P2 = e0.00151                = 1.0015        P2 = 0.9989 atm So the pressure has to be lowered to change the boiling point.Density is not required in this equation.                = 1.0015        P2 = 0.9989 atm So the pressure has to be lowered to change the boiling point.Density is not required in this equation.

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