Final answer:
The concentration of hydronium ions [H3O+] in an 8.64×10⁻³ M HBr solution is also 8.64×10⁻³ M, since HBr is a strong acid that completely dissociates in water.
Step-by-step explanation:
To calculate the concentration of hydronium ions ([H3O+]) in an 8.64×10⁻³ M HBr solution, we must consider that hydrobromic acid (HBr) is a strong acid. This means it completely dissociates in water. Therefore, the concentration of [H3O+] will be equal to the concentration of the HBr solution.
[H3O+] for an 8.64×10⁻³ M HBr solution is also 8.64×10⁻³ M because the acid dissociates completely, releasing an equivalent amount of H3O+ ions into the solution.