The total volume of a new house is $800 \mathrm{m}^{3} .$ Before the heat is turned on, the air temperature inside is $10^{\circ} \mathrm{C},$ and the relative humidity is 50 percent. After the air is warmed to $20^{\circ} \mathrm{C}$, how much water vapor must be added to the air to make the relative humidity 50 percent?