Something with a molar mass of 38 g/mol means that the compound's elements have atomic masses that add up to 38. Atomic masses are given on the periodic table.
One possible formula could be F₂, which is the molecular formula for Fluorine because it is diatomic, and has to be bonded to something. The atomic mass of Fluorine is about 19, so two of those would make a molar mass of 38.
Negative is it can't be broken down and makes the air polluted. Positive, it can be re used again and again.