The correct formula to use for this question is Clausius Clapeyron equation which states that: ln [P2/P1] = [- Change in Hvap/R] [1/T2 - 1/T1]
P1 is the normal pressure, P2 is the given pressure, R is the ideal gas law constant, T1 is the normal temp of water in kelvin, T2 is the value we want to calculate. Hvap is the heat of vaporisation.
using the above equation: In [0.5/1] = 40.7/8.314] [1/T2 - 1/373]
- 0.69 = [- 4.9] [1/T2 - 0.00268] = 89.9.
therefore at 0.5 atmospheric pressure water will boil at 89.9 degree celcius or 362.9 at kelvin temprature.
