posted by sand .
The temperature in a room is kept at a constant 20 °C but the humidity is only 20%. If the room has a volume of 30m³, and the room’s door and windows closed, how many kilograms of water must be put into the vaporizer to increase the humidity from 20% to 85% ( the molecular weight of water is 18)
You must be referring to the relative humidty (R.H.), not the absolute humidity.
You need to know the saturation pressure of water at 20 C. It is easy to look up. From that, you can calculate how much water vapor the room would contain at 100% relative humidity. You will need to use the ideal gas law.
Then calculate the water mass difference between 85% R.H. and 20% R.H. cases.
Thanks for your guidance