Vapour pressure of water - wikipedia Vapour pressure of water The vapour pressure of water is the pressure at which water vapour is in thermodynamic equilibrium with its condensed state. At higher pressures water would condense. The water vapour pressure is the partial pressure of water vapour in any gas mixture in equilibrium with solid or liquid water. As for other substances, water vapour pressure is a function of temperature and can be determined with the Clausius -- Clapeyron relation. Vapour pressure of water ( 0 -- 100 ° C ) T, ° C T, ° F P, kPa P, torr P, atm 0 32 0.6113 4.5851 0.0060 5 41 0.8726 6.5450 0.0086 10 50 1.2281 9.2115 0.0121 15 59 1.7056 12.7931 0.0168 20 68 2.3388 17.5424 0.0231 25 77 3.1690 23.7695 0.0313 30 86 4.2455 31.8439 0.0419 35 95 5.6267 42.2037 0.0555 40 104 7.3814 55.3651 0.0728 45 113 9.5898 71.9294 0.0946 50 122 12.3440 92.5876 0.1218 55 131 15.7520 118.1497 0.1555 60 140 19.9320 149.5023 0.1967 65 149 25.0220 187.6804 0.2469 70 158 31.1760 233.8392 0.3077 75 167 38.5630 289.2463 0.3806 80 176 47.3730 355.3267 0.4675 85 185 57.8150 433.6482 0.5706 90 194 70.1170 525.9208 0.6920 95 203 84.5290 634.0196 0.8342 100 212 101.3200 759.9625 1.0000 Contents 1 Approximation formulas 1.1 Accuracy of different formulations 2 Numerical approximations 3 Graphical pressure dependency on temperature 4 See also 5 References 6 Further reading 7 External links Approximation formulas There are many published approximations for calculating saturated vapour pressure over water and over ice. Some of these are ( in approximate order of increasing accuracy ) : P = exp ⁡ ( 20.386 − 5132 T ), ( E q. 1 ) ( displaystyle P = exp left ( 20.386 - ( \frac ( 5132 ) ( T ) ) \right ), ( Eq. 1 ) ) where P is the vapour pressure in mmHg and T is the temperature in kelvins. The Antoine equation log 10 ⁡ P = A − B C + T, ( displaystyle log _ ( 10 ) P = A - ( \frac ( B ) ( C + T ) ), ) where the temperature T is in degrees Celsius ( ° C ) and the vapour pressure P is in Torr. The constants are given as T, ° C T, ° C 8.07131 1730.63 233.426 99 8.14019 1810.94 244.485 100 374 The August - Roche - Magnus ( or Magnus - Tetens or Magnus ) equation, as described in Alduchov and Eskridge ( 1996 ). Equation 21 in provides the coefficients used here. See also discussion of Clausius - Clapeyron approximations used in meteorology and climatology. P = 0.61094 exp ⁡ ( 17.625 T T + 243.04 ), ( displaystyle P = 0.61094 exp left ( ( \frac ( 17.625 T ) ( T + 243.04 ) ) \right ), ) where temperature T is in ° C and vapour pressure P is in kilopascals ( kPa ) The Tetens equation P = 0.61078 exp ⁡ ( 17.27 T T + 237.3 ), ( displaystyle P = 0.61078 exp left ( ( \frac ( 17.27 T ) ( T + 237.3 ) ) \right ), ) where temperature T is in ° C and P is in kPa The Buck equation. P = 0.61121 exp ⁡ ( ( 18.678 − T 234.5 ) ( T 257.14 + T ) ), ( displaystyle P = 0.61121 exp left ( left ( 18.678 - ( \frac ( T ) ( 234.5 ) ) \right ) left ( ( \frac ( T ) ( 257.14 + T ) ) \right ) \right ), ) where T is in ° C and P is in kPa. The Goff - Gratch ( 1946 ) equation. Accuracy of different formulations Here is a comparison of the accuracies of these different explicit formulations, showing saturation vapour pressures for liquid water in kPa, calculated at six temperatures with their percentage error from the table values of Lide ( 2005 ) : T ( ° C ) P ( Lide Table ) P ( Eq 1 ) P ( Antoine ) P ( Magnus ) P ( Tetens ) P ( Buck ) P ( Goff - Gratch ) 0 0.6113 0.6593 ( + 7.85 % ) 0.6056 ( - 0.93 % ) 0.6109 ( - 0.06 % ) 0.6108 ( - 0.09 % ) 0.6112 ( - 0.01 % ) 0.6089 ( - 0.40 % ) 20 2.3388 2.3755 ( + 1.57 % ) 2.3296 ( - 0.39 % ) 2.3334 ( - 0.23 % ) 2.3382 ( + 0.05 % ) 2.3383 ( - 0.02 % ) 2.3355 ( - 0.14 % ) 35 5.6267 5.5696 ( - 1.01 % ) 5.6090 ( - 0.31 % ) 5.6176 ( - 0.16 % ) 5.6225 ( + 0.04 % ) 5.6268 ( + 0.00 % ) 5.6221 ( - 0.08 % ) 50 12.344 12.065 ( - 2.26 % ) 12.306 ( - 0.31 % ) 12.361 ( + 0.13 % ) 12.336 ( + 0.08 % ) 12.349 ( + 0.04 % ) 12.338 ( - 0.05 % ) 75 38.563 37.738 ( - 2.14 % ) 38.463 ( - 0.26 % ) 39.000 ( + 1.13 % ) 38.646 ( + 0.40 % ) 38.595 ( + 0.08 % ) 38.555 ( - 0.02 % ) 100 101.32 101.31 ( - 0.01 % ) 101.34 ( + 0.02 % ) 104.077 ( + 2.72 % ) 102.21 ( + 1.10 % ) 101.31 ( - 0.01 % ) 101.32 ( 0.00 % ) A more detailed discussion of accuracy and considerations of the inaccuracy in temperature measurements is presented in Alduchov and Eskridge ( 1996 ). The analysis here shows the simple unattributed formula and the Antoine equation are reasonably accurate at 100 ° C, but quite poor for lower temperatures above freezing. Tetens is much more accurate over the range from 0 to 50 ° C and very competitive at 75 ° C, but Antoine 's is superior at 75 ° C and above. The unattributed formula must have zero error at around 26 ° C, but is of very poor accuracy outside a very narrow range. Tetens ' equations are generally much more accurate and arguably simpler for use at everyday temperatures ( e.g., in meteorology ). As expected, Buck 's equation for T > 0 ° C is significantly more accurate than Tetens, and its superiority increases markedly above 50 ° C, though it is more complicated to use. The Buck equation is even superior to the more complex Goff - Gratch equation over the range needed for practical meteorology. Numerical approximations For serious computation, Lowe ( 1977 ) developed two pairs of equations for temperatures above and below freezing, with different levels of accuracy. They are all very accurate ( compared to Clausius - Clapeyron and the Goff - Gratch ) but use nested polynomials for very efficient computation. However, there are more recent reviews of possibly superior formulations, notably Wexler ( 1976, 1977 ), reported by Flatau et al. ( 1992 ). Graphical pressure dependency on temperature Vapour pressure diagrams of water ; data taken from Dortmund Data Bank. Graphics shows triple point, critical point and boiling point of water. See also Dew point Gas laws Molar mass Clausius -- Clapeyron relation Goff - Gratch equation Antoine equation Tetens equation Arden Buck equation Lee -- Kesler method References Jump up ^ Lide, David R., ed. ( 2004 ). CRC Handbook of Chemistry and Physics, ( 85th ed. ). CRC Press. pp. 6 -- 8. ISBN 978 - 0 - 8493 - 0485 - 9. ^ Jump up to : Alduchov, O.A. ; Eskridge, R.E. ( 1996 ). `` Improved Magnus form approximation of saturation vapor pressure ''. J. Applied Meteorology. 35 ( 4 ) : 601 -- 9. Bibcode : 1996JApMe... 35... 601A. doi : 10.1175 / 1520 - 0450 ( 1996 ) 035 < 0601 : IMFAOS > 2.0.CO ; 2. Jump up ^ Goff, J.A., and Gratch, S. 1946. Low - pressure properties of water from − 160 to 212 ° F. In Transactions of the American Society of Heating and Ventilating Engineers, pp 95 -- 122, presented at the 52nd annual meeting of the American Society of Heating and Ventilating Engineers, New York, 1946. Jump up ^ Lowe, P.R. ( 1977 ). `` An approximating polynomial for the computation of saturation vapor pressure ''. J. Applied Meteorology. 16 : 100 -- 4. Bibcode : 1977JApMe... 16... 100L. doi : 10.1175 / 1520 - 0450 ( 1977 ) 016 < 0100 : AAPFTC > 2.0.CO ; 2. Jump up ^ Wexler, A. ( 1976 ). `` Vapor pressure formulation for water in range 0 to 100 ° C. A revision '' ( PDF ). J. Res. Natl. Bur. Stand. 80A ( 5 - 6 ) : 775 -- 785. doi : 10.6028 / jres. 080a. 071. Jump up ^ Wexler, A. ( 1977 ). `` Vapor pressure formulation for ice '' ( PDF ). J. Res. Natl. Bur. Stand. 81A ( 1 ) : 5 -- 20. doi : 10.6028 / jres. 081a. 003. Jump up ^ Flatau, P.J. ; Walko, R.L. ; Cotton, W.R. ( 1992 ). `` Polynomial fits to saturation vapor pressure ''. J. Applied Meteorology. 31 ( 12 ) : 1507 -- 13. Bibcode : 1992JApMe... 31.1507 F. doi : 10.1175 / 1520 - 0450 ( 1992 ) 031 < 1507 : PFTSVP > 2.0.CO ; 2. Further reading `` Thermophysical properties of seawater ''. Matlab, EES and Excel VBA library routines. MIT. 20 February 2017. Garnett, Pat ; Anderton, John D ; Garnett, Pamela J ( 1997 ). Chemistry Laboratory Manual For Senior Secondary School. Longman. ISBN 0 - 582 - 86764 - 9. Murphy, D.M. ; 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 -- 65. Bibcode : 2005QJRMS. 131.1539 M. doi : 10.1256 / qj. 04.94. Speight, James G. ( 2004 ). Lange 's Handbook of Chemistry ( 16th ed. ). McGraw - Hil. ISBN 0071432205. External links Vömel, Holger ( 2016 ). `` Saturation vapor pressure formulations ''. Boulder CO : Earth Observing Laboratory, National Center for Atmospheric Research. 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Question: vapor pressure of water at 20 c kpa
Answer: 2.3388