The only chemical elements that form stable homonuclear diatomic molecules at standard temperature and pressure (STP) are the gaseshydrogen (H2), nitrogen (N2), oxygen (O2), fluorine (F2), and chlorine (Cl2). The answer must be one of the abovementioned element.
38.00/2 = 19 g/mol
Therefore, the gas is fluorine since it has an atomic weight 19 g/mol.