Find the mass of all molecules leaving one square centimetre of water surface per second into a saturated water vapour above it at a temperature `t = 100 ^@C`. It is assumed that `eta = 3.6 %` of all water vapour molecules falling on the water surface are retained in the liquid phase.

Explanation

In equilibrium the number of "liquid" molecules evaporoting must equals the number of "vapour" molecules condensing. By kinetic theory, this number is
`eta xx (1)/(4) n lt v gt = eta xx (1)/(4) n xx sqrt((8)/(pi)(kT)/(m))`
Its mass is
`mu = m xx eta xx n xx sqrt((kT)/(2 pi m)) = eta n k T sqrt((m)/(2 pi kt))`
`=eta p_0 sqrt((M)/(2 pi RT)) = 0.35 g//cm^2.s.`
where `p_0` is atmospheric pressure and `T = 373 K` and `M` = molecular weight of water.