The total volume of a new house is 800 m3. Before the heat is turned on, the air temperature inside...

The total volume of a new house is 800 m³. Before the heat is turned on, the air temperature inside is 10°C, and the relative humidity is 50%. After the air is warmed to 20°C, how much water vapor must be added to the air to make the relative humidity 50%? 
m = [removed]  kg