Final answer:
Most H+ ions come from the dissociation of water molecules, but stronger acids can also contribute a significant amount of H+ ions.
Step-by-step explanation:
In most cases, the majority of H+ ions, also known as hydrogen ions, come from the dissociation (ionization) of water molecules. When water molecules dissociate, they form equal numbers of H+ ions and hydroxide (OH-) ions. The H+ ions then attract to un-ionized water molecules, forming hydronium ions (H3O+).
For weak acids, where the concentration of the acid is high, the amount of H+ ions from water may be very small compared to the amount from the acid.
In summary, most H+ ions come from the dissociation of water molecules, but for strong acids, the majority of H+ ions come from the acid itself.