The heaviest gas out of H₂S, NH₃, CO, and SO₂ is SO₂ (sulfur dioxide).

Here's why:

* Molecular Weight: The heavier a molecule is, the denser its gas will be. Here are the molecular weights:

* H₂S (hydrogen sulfide) - 34 g/mol

* NH₃ (ammonia) - 17 g/mol

* CO (carbon monoxide) - 28 g/mol

* SO₂ (sulfur dioxide) - 64 g/mol

* Density: While not directly provided, the density of a gas is directly proportional to its molecular weight. SO₂ has the highest molecular weight, meaning it will have the highest density.

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