Someone please help me!!!!!!!

Hydrogen chloride gas is escaping from a train wreck.
Firefighters walking along the tops of the tank cars are not affected by the leaking
gas. Firefighters walking in low lying areas (close to the ground) are overcome by
hydrogen chloride fumes.
Explain, using concepts we've learned, why this could be happening to the
firefighters in the different locations.

Hydrogen Chloride Gas is heavier than air and sinks to the ground after being released. Firefighters closer to the ground will be subjected to significantly greater concentrations of hydrogen chloride gas than the firefighters walking on top of railcars.

Explanation:

The molar mass of air is 28.97 g/mol. Air is approximately 78% N2, which has a molar mass of 28.014 g/mol and 21% oxygen, which has a molar mass of 32 g/mol; the remaining constituents of air include CO2 and other trace amounts of gases.

The molar mass of HCl is 36.458 g/mol.

Because HCl is heavier than air, it sinks toward the ground, displacing the lighter air.

The height of railway cars is reported to vary between 13.5 ft to 15.5 ft. So firefighters on tank cars are generally above the HCl gas (although localized atmospheric conditions could cause the HCl to move higher up in the air column).