Hydrogen sulfide, H2S (molar mass 34 g/mol), which smells like rotten eggs,
and hydrogen cyanide HCN (molar mass 27 g/mol), which smells like bitter almonds.

Both gases are poisonous. If you were in a room ten meters away from a vent that is releasing both gases at the same time and in the same amount, which would you smell first, and why?

hydrogen cyanide HCN (molar mass 27 g/mol),

Explanation:

The hydrogen cyanide gas, HCN will diffuse faster and would be the first to be perceived some distance away.

We can look into this problem using Graham's law of diffusion. The law states that "the rate of diffusion or effusion of a gas is inversely proportional to the square root of its molecular mass or density at constant temperature and pressure".

Since HCN has a lower molar mass, it will diffuse faster than H₂S.