$4)$ Water evaporates from an open bowl at $349$ K at the rate of $4\mathrm{.}11$x $10^3$ kg per m$^2$

per second. Assume ideal gas and neglect the partial pressure of vapor in the surrounding atmosphere. $($a$)$ What is the effective gas$-$film thickness?

$($b$)$ The water is replaced by ethanol at $343$ K$.$ What will be its rate of cvaporation in kg per m$^2$ per second if the film thickness is unchanged?

$($c$)$ At the surface of the ethanol, what proportion of the total mass transfer will then be attributable to bulk flow? DATA: Vapor pressure of water at $349$ K$=34$ mmHg Vapor pressure of ethanol at $343$ K$=544$ mmHg Diffusivity of water vapor in air $=2\mathrm{.}6$ x $10^5$

m$^2/$s Diffusivity of ethanol in air $=1\mathrm{.}2$ x $10^5$ m$^2/$s Density of mercury $=13\mathrm{.}6$ g$/$cm$^3$