[H+] and [OH-] Relationship


  • Sometimes water collides with another water molecule with enough force to transfer hydrogen.
  • The result is a hydroxide ion (OH-) that is a base and hydronium ion (H3O+), which is an acid.
  • This is known as self-ionization of water.


  • In pure water at 25oC the concentration of [H+] and the concentration of [OH-] are each only 1.0 x 10-7 M.
  • what this means is the concentration of [H+] and [OH-] are equal and is described as a neutral solution

Water constant

  • In most cases [H+] and [OH-] are interdependent meaning that when [H+] increases [OH-] decreases and vis versa.
  • For aqueous solutions, the product of hydrogen ion concentration and hydroxide ion concentration equals 1.0 x 10-14 
  •   [H+] x [OH-] = 1.0 x 10-14
  • The product of the concentrations of the hydrogen ions and the hydroxide ions in water is called the ion-product constant for water (Kw
  •   Kw = [H+] x [OH-] = 1.0 x 10-14

Acids and Bases Numerically

  • Not all solutions are neutral, when this happens the hydrogen ion concentration is greater than the hydroxide ion concentration and is known as an acidic solution. Which means that [H+] is greater than 1.0 x 10-7 M  
  • A base is a solution with a larger hydroxide ion concentration than the hydrogen ion concentration. Which means that [H+] is less than 1.0 x 10-7 M.  Basic solutions are known as alkaline solutions.

Example Prob(Copy all)

  • Ex prob
  • If the [H+] is 1.0 x 10-5 M, is this solution acidic, basic, or neutral? What is the [OH-] of this solution? 
  • If the concentration of [H+] is more than 1.0 x 10-7 then it is an acidic solution 
  • To determine the [OH-] use the Kw information and substitute the knowns. 
  •   Kw = [H+] x [OH-] = 1.0 x 10-14
  • We know Kw = 1.0 x 10-14 and [H+] = 1.0 x 10-5 so rearrange for [OH-] and solve 
  •   Kw=  [OH-]       so      1.0 x 10-14   =  1.0 x 10-9 M
  •   [H+]                          1.0 x 10-5 M

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