# Why are logarithms used in chemistry

In order not to always have to calculate with power numbers, logarithmic quantities are introduced. The pH value is expressed as the negative decadic logarithm of the H.3O+-Ion concentration defined.

 pH = - log [H3O+]

The pH scale is based on this definition. It ranges from pH = 0 ([H.3O+] = 1 mol / l = 10 0 mol / l) to pH = 14 ([H3O+] = 10 -14 minor). If the pH value in a solution drops by one unit (by 1.0), this means, based on the logarithmic scale, that the hydronium ion concentration has increased by a factor of 10 and vice versa. The pOH value is analogous as the negative decadic logarithm of the OH -Ion concentration defined.
pOH = - log [OH ]
pH and pOH are related to the ionic product of water. If you take the negative logarithm of the ion product, you get
 - lied KW.  =   - lied ([H3O+]·[OH–])  =   - lied 10 -14   = pH + pOH = 14

This shows how practical the logarithmic division is. If the pH value is known, the hydroxide ion concentration can be calculated very easily using pOH = 14 - pH. The pOH scale runs in the opposite direction to the pH scale. An example: At a pH of 5, the pOH is 9, the hydroxide ion concentration is then [OH] = 10-9 minor.

 pH ranges in various foods and body fluids: