In a solution, hydrogen ions normally bond with molecules of water, forming H3O+ (hydronium) ions. Thus, the concentration of the hydronium ions will be the same as the concentration of hydrogen ions, which is related to the pH of a solution according to the following equation:
pH = -log[H+] = -log[H3O+]
This equation can be solved for the concentration of hydronium ions:
[H3O+] = 10-pH
Thus, for a solution with a pH of 3, the concentration of hydronium ions will be 10-3 = 0.001 moles/liter, and for a solution with a pH of 8, the concentration of hydronium ions will be 10-8 = 0.00000001 moles/liter.