Final answer:
Each H2 molecule dissociates into two H atoms, so the molar concentration of H atoms at equilibrium is twice the concentration of H2, which calculates to 0.56 M.
Step-by-step explanation:
The molar concentration of hydrogen atoms (H atoms) at equilibrium when the equilibrium concentration of hydrogen gas (H2) is 0.28 M can be determined by understanding that each molecule of H2 dissociates into two H atoms. Since H2 exists as a diatomic molecule, the molar concentration of H atoms would be twice the molar concentration of H2 molecules at equilibrium. The calculation is as follows:
[H atoms] = 2 × [H2]
Therefore, the molar concentration of H atoms at equilibrium is:
[H atoms] = 2 × 0.28 M = 0.56 M
This value has two significant figures, as requested