H₃O ions are when water is protonated
H₂O + H⁺ --> H₃O⁺
HCl is a strong acid therefore complete ionisation takes place.
HCl ---> H⁺ + Cl⁻
1 mol of HCl dissociates to give out 1 mol of H⁺
since H⁺ together with water forms H₃O⁺ ions,
[HCl] = [H₃O⁺]
therefore concentration of [H₃O⁺] is 0.0005 M