Question 6.3.1: Composition of a Saturated Gas–Vapor System Air and liquid w......

Since the gas and liquid are in equilibrium, the air must be saturated with water vapor (if it were not, more water would evaporate), so that Raoult’s law may be applied:

y_{H_{2}O}=p^{*}_{H_{2}O} (75°C)/P

From Table B.3 in Appendix B, p^{*}_{H_{2}O}(75°C) = 289 mm Hg. Consequently,

\begin{matrix}y_{H_{2}O}=\frac{289  mm  Hg}{760  mm  Hg} = \boxed{0.380  \frac{mol  H_{2}O}{mol} }\\\\ y_{dry  air}= 1- y_{H_{2}O}= \boxed{0.620  \frac{mol  dry  air}{mol}}\end{matrix}