Answer
It's greater in A.
Work Step by Step
From appendix G, the vapor pressure of water is:
$@20°C: 17.5\ mmHg$
$@0°C: 4.6\ mmHg$
Partial pressures:
A) $45/100×17.5=7.88\ mmHg$
B) $95/100×4.6=4.37\ mmHg$
Mol fraction of water vapor:
A) $7.88/760×100\%=1.037\%$
$1.037/100×18.015=0.187\ g/mol\ air$
B) $4.37/760×100\%=0.575\%$
$0.575/100×18.015=0.104\ g/mol\ air$
Molar volume of air:
$V/n=RT/P$
A) $V/n=0.082×293\ K/1\ atm=24.04\ L/mol$
B) $V/n=0.082×273\ K/1\ atm=22.40\ L/mol$
Thus, the mass of water per liter of air is:
A) $0.187\ g/mol\div 24.04\ L/mol=0.0078\ g/L$
B) $0.104\ g/mol\div 22.40\ L/mol=0.0046\ g/L$
It's greater in A.
