# Issue with evaporation rate of water

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)

b)

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

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?

• 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?? – DrMoishe Pippik Jun 20 at 20:17
• Yes, that is a typo. I'll fix it. – JD_PM Jun 20 at 20:23
• What kind of exercise are they assigning! Basically this asks for "how many second are in one hour" in a convoluted way. – Alchimista Jun 21 at 10:18
• 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/… – JD_PM Jun 21 at 15:05
• 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. – JD_PM Jun 21 at 15:06