Reference Guides

Calculating pH, pOH, H3O+, and OH-


“pH” is an abbreviation for “potential of hydrogen”. It is a unit of measurement that stands for the concentration of hydrogen ions that are within a solution. When an acid or base is mixed with water, the compound disassociates into ions. In acids one of the ions is a hydrogen ion (H+), In acids, one of the ions is a hydroxide ion (OH-). The concentration of ions is commonly described by the pH scale as a numeric value.


In this guide, we will be going over a few different ways to find pH, pOH, acid concentration, and base concentration[OH].


  • Given [H+] = 4.1 x 10-4M, find the following: We have the concentration and will attempt to find:
    • pH
    • pOH
    • [OH]


    To find the pH we will use the following formula using the given acid concentration: pH = – log (4.1 x 10-4M)
    Note: The number of sig figs will be the number of decimal places pH and pOH should be rounded to
    Answer: pH = 3.39
    As we have found the pH we can now use the following formula to find the pOH: 3.39 + pOH = 14
    After subtracting 3.39 from both the pH and 14 we will get the pOH.
    Answer: (3.39 – 3.39)+(14 – 3.39)= pOH 10.61
    As we have found the pOH, we will now go ahead with finding the base concentration [OH]. To do this can use the following formula: [OH] = 10-10.61
    Answer: [OH] = 2.5 x 10-11M


Hydrogen Ions are present in all aqueous solutions. The concentration of these ions in a solution is important in determining the properties of a solution and the chemical behaviors of its other solutes.
A solution is considered neutral if it has equal concentrations of hydronium and hydroxide ions and acidic if it has more hydronium ions than hydroxide ions; and basic if it contains a lesser concentration of hydronium ions than hydroxide ions.

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