Assume that the water vapor is an ideal gas. So,
PV = nRT
For conversion, 760 mmHg = 101325 Pa and 1,000 L = 1 m³
(187.5 mmHg)(101325 Pa/760 mmHg)(5 L)(1 m³/1,000 L) = n(8.314 m³Pa/molK)(65+273 K)
Solving for n,
n = 0.0445 mole water
Since the molar mass of water is 18 g/mol,
Mass of water vaporized = 0.0445*18 = 0.8 g water vaporized
Hence,
Percentage of water vaporized = 0.8/1.2 * 100 = 66.7%