The relative mass formula for hydrogen gas (H₂), is simply:

2

Here's why:

* Hydrogen's Atomic Mass: Hydrogen (H) has an atomic mass of approximately 1 atomic mass unit (amu).

* Diatomic Molecule: Hydrogen gas exists as a diatomic molecule, meaning two hydrogen atoms bond together (H₂).

* Relative Mass Calculation: Since each hydrogen atom has a relative mass of 1, the relative mass of two hydrogen atoms (H₂) is 1 + 1 = 2.

Therefore, the relative mass formula of hydrogen gas is 2.