Vapor Pressure of Water

Vapour pressure diagrams of Water, data taken from Dortmund Data Bank

The vapour pressure of water is the vapour pressure (or equilibrium/saturation pressure) of water, i.e., the pressure exerted by water at a specific temperature. It is important in many experiments, particularly experiments relating to gases. A common classroom experiment in which the vapour pressure at various temperatures table is used is when trying to find the molar mass of butane. The experiment is done by releasing butane gas from a cigarette lighter underwater into glass tubing. By calculating the partial pressure of the gas and subtracting the atmospheric pressure on the day that the experiment was conducted, it is possible to obtain an accurate result. This can be done with other gases with the same process.

Some equations used to approximate the vapour pressure of water are (in order of increasing accuracy):

  • A very simple equation:
$ P = \exp(20.386-5132/T) \, $
where P is the vapour pressure (mmHg) and T is the temperature in kelvins
$ \log_{10}P = 7.96681 - \frac{1668.21}{228.0 + T} $
or transformed into this temperature-explicit form:
$ T = \frac{1668.21}{7.96681 - \log_{10}P} - 228.0 $
where the temperature T is in degrees Celsius and the vapour pressure P is in mmHg.

See alsoEdit


  • Garnett, Pat; Anderton, John D; Garnett, Pamela J Chemistry Laboratory Manual For Senior Secondary School, Longman. 1997. ISBN 0582867649. 
  • Murphy, D. M. and Koop, T. (2005): Review of the vapour pressures of ice and supercooled water for atmospheric applications, Quarterly Journal of the Royal Meteorological Society 131(608): 1539-1565. doi:10.1256/qj.04.94

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