It is a fact (i) that most elemental gases are DIATOMIC; it is a fact (ii) that mass is always conserved in a chemical reaction.
Most of the elemental gases,
Knowledge of the molecularity of the elemental gases would be assumed for an A level student, and it certainly would be expected of a 1st year college student.
Now chemical reactions follow experimental result; experimental result does not follow our depiction of the equation. For the reaction of oxygen and hydrogen, we know that 1/2 an equiv of dioxygen reacts with one equiv of dihydrogen under normal conditions to give one equiv of water:
If I wanted to, I could double the entire equation to get rid of the half-integral coefficients.
The point is that these equations are representations of chemical reality. Now these reactions certainly conserve mass: in
Given certain conditions, however, we could in fact produce hydrogen peroxide under so-called reducing conditions:
This would be admittedly hard to do, nevertheless, we can still conceive of the reaction as a formality, and we can certainly estimate the thermodynamic parameters involved in the reaction. In the reaction
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