In pure water, a small number of the water molecules ionize, resulting in hydronium and hydroxide ions. A hydronium ion is a water molecule that has taken on an extra proton and a positive charge, and thus has the formula H3O+ instead of H2O. The presence of a large number of hydronium ions lowers the pH of a water-based solution. pH is a measure of the acidity of a solution and is a logarithmic reflection of the amount of hydronium ions present in the solution. pH measurements can range from 0 to 14. You can use this information to calculate the theoretical concentration of hydronium ions in any solution.
Note the pH of the solution in question. Usually you can read the label of the solution or look up the pH of common substances in a chemistry book or online reference. If it is an unknown solution with an unknown pH, use a pH meter or conduct a chemical titration to determine its pH.
Solve the pH equation for the "hydronium ion concentration" variable.
pH = - log (Hydronium Ion Concentration), so
Hydronium Ion Concentration = 10^(-pH)
(^ = symbol that means to the power of)
Plug the pH value of your solution into the equation to reveal the concentration of hydronium ions in the solution. For example, consider a solution with a pH of 2.
Hydronium Ion Concentation = 10^-2 = 0.01 moles/liter
There are 0.01 moles of hydronium ion per liter of your solution.