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Homework Help: Thermal Physics - Dalton's Law etc

  1. Aug 11, 2009 #1
    Inside the leaf of a plant, water vapour passes from the liquid phase to the vapour phase at the walls of the mesophyll cells, as shown in the figure (N/A due to copyright). The water vapour then diffuses through the intercellular air spaces and eventually exits the leaf through the stomatal pores. The diffusion constant for water vapor in air is D = 2.4 x 10−5 m2s−1. A stomatal pore has a cross-sectional area A = 6.8 x 10−11 m2 and a length L = 7.0 x 10−5 m. The plant is being propagated in a controlled environment: T = 17 °C, relative humidity 61 %. The saturated vapour pressure of water at 17 °C is 1.93 kPa.

    (a) Assuming that the air around the plant is an ideal gas, what is the concentration of water molecules per cubic meter in the air? __________ molecules/m³

    (b) Given that the molar mass of water is 18.0 g mol−1, what is the concentration of water in the air in kg/m³? __________ kg/m³

    2. Relevant equations - don't know...

    I know that the partial pressure of water vapour is 1.18kPa which can be worked out by using the saturated vapour pressure of water and the relative humidity ( x/1.93 = 0.61 , x = 1.18kPa )
    But what do you do with it? I think its got something to do with the total pressure and etc but don't know exactly how
    Please help me
  2. jcsd
  3. Aug 14, 2009 #2


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    Staff Emeritus
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    Homework Helper

    Hint: ideal gas law. You're correct that p for the water is 1.18 kPa.
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