What is the calculated atmospheric pressure in mm Hg if the boiling point of water boils at 81 C?

Question

What is the calculated atmospheric pressure in mm Hg if the boiling point of water boils at 81 C?

Answer 1

One has to employ the Clausius-Clapeyron equation:

ln(p/p0)= (Hv/R)*(1/T0-1/T1)

Hv: Enthalpy of vaporisation ( 2200 J/g)

R: mass related Avogadro constant (=8.3¹4J/mol K/18 g/mol )

p: unknown pressure at known temperature T

p0: known pressure at known temperature T0  (p0= 1013 mbar at T= 373 K)

Answer 2

You cannot use gas laws to calculate the boiling point of water at different pressures, according to my knowledge.

I have checked a web page in Google, which shows that for water to boil at 81°C the atmospheric pressure will be 292mm Hg

I used the site www.primogrill.com/boiling.htm

Answer 3

g*y-Lussac's Law states that P1/T1 = P2/T2 where P1 = initial pressure, T1 = initial absolute temperature, P2 = new pressure and T2 = new absolute temperature. At 760 mm Hg, water boils at 100 C (373.13K). So:

P1 = 760

T1 = 373.13

T2 = 81 + 273.13 = 354.13

P1/T1 = P2/T2

     P2 = P1 x T2/T1

     P2 = 760 x 354.13/373.13

     P2 = 721.3

Therefore the atmospheric pressure when the boiling point of water is 81 C is 721.3 mm Hg.