The evaporation rate of water (kg/s) is given by:

$$g = \frac{P}{P_o} AΘ$$

Where $P$ is given in Pascals and $A$ is the surface area (in $m^2$) and $Θ$ is the empirical evaporation coefficient:

$$Θ = 12v + 16$$

where $v$ the wind speed (perpendicular to the water's surface).

a) Calculate the evaporated mass of water in one hour when $v = 0$.

b) Calculate the enthalpy and entropy difference within $1$ hour when $v = 0$.

What I have tried:

a) I understand $P$ to be the partial pressure of water vapor in moist air and $P_o$ to be the atmospheric pressure of moist air.

I am not given neither the values for these pressures nor the surface area... How am I supposed to calculate the evaporated mass without such data? (It's stated that all the necessary information is provided)


$$\Delta H = \Delta U + P\Delta V$$

Holding fixed the number of molecules:

$$dH = TdS+ VdP$$

But again I don't have enough information...

Are there flaws in this exercise or am I missing something?

  • 1
    $\begingroup$ You are correct, you cannot calculate the answer in absolute terms, e.g. kg, so you must answer the question in terms of the pressures, areas and the constant 16 (since v drops out). How does theta become gamma, though? Typo?? $\endgroup$ Jun 20, 2019 at 20:17
  • $\begingroup$ Yes, that is a typo. I'll fix it. $\endgroup$
    – JD_PM
    Jun 20, 2019 at 20:23
  • 1
    $\begingroup$ What kind of exercise are they assigning! Basically this asks for "how many second are in one hour" in a convoluted way. $\endgroup$
    – Alchimista
    Jun 21, 2019 at 10:18
  • $\begingroup$ Yeah, this exercise is not really useful. I am interested in doing exercises in vapor-gas equilibrium (saturated vapor), which is described by the Clausius–Clapeyron relation: en.wikipedia.org/wiki/… $\endgroup$
    – JD_PM
    Jun 21, 2019 at 15:05
  • $\begingroup$ So if anyone has a set of exercises on it, could you please share it? I have more exercises on it but are confusing like above. $\endgroup$
    – JD_PM
    Jun 21, 2019 at 15:06


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