What is the difference in hydrogen ion concentration between hydrochloric acid and a lemon

Answer is: hydrogen ion concentration (H⁺) is greater in hydrochloric acid than a lemon with same concentration of both solutions.

Strong acids (in this example hydrochloric acid) completely ionize in water and give large amount of hydrogen ions (H⁺) : HCl (aq) → H⁺ (aq) + Cl⁻ (aq).

Weak acids (lemon has citric acid) partially ionize in water and give only a few hydrogen ions (H⁺), in the solution there molecules of acid and ions.