Answer
0.91 kg.
Work Step by Step
The volume and temperature of the water vapor are known. The saturated vapor pressure of water, from Table 13–3, is 3170 Pa, and the relative humidity is $55\%$, so the partial pressure of water is 55 percent of 3170 Pa.
Use the ideal gas law to find the mass.
$$n=\frac{PV}{RT}=\frac{(0.55)(3170Pa)(5.0m)(6.0m)(2.4m)}{(8.314)(273.15+25K)}=50.64\;mol$$
The mass is (50.64 moles)(0.018 kg/mole)=0.91 kg.