36. Solving a chemistry problem

4. When monitoring air pollution, the detector for natural gas methane CH4 should be located at a distance of 10-20 cm from the ceiling, and for carbon monoxide CO at a distance of 1.5-1.8 m from the floor. Why? Prove by calculations that these gases are lighter or heavier than air? Calculate the mass of 1 liter of each of these gases under normal conditions.