Hydronium-Hydroxide Balance
Home Up Self-Ionization of Water Definition of pH Hydronium-Hydroxide Balance Kw Calculating H-OH Ion Concentrations Calculating pH and pOH

 

Hydronium/Hydroxide Balance

When an acid dissolves in water, additional H3O+ is formed, increasing the concentration of H3O+. For example, the concentration of H3O+ might be increased from 10-7 M up to 10-5 M. That is 100 times more concentrated. Note that the pH, the number behind the negative sign in the exponent, changes from 7 to 5. This is why acidic solutions have pH values lower than 7.

The acidity or basicity of a solution is related to the relative concentrations of H3O+ and OH-. If the concentration of H3O+ is more than the concentration of OH-, the solution is acidic. If the concentration of OH- is more than the concentration of H3O+, then the solution is basic. If the concentrations of H3O+ and OH- are equal to one another, the solution is neutral.
[H3O+] > [OH-] acidic
[OH-] > [H3O+] basic
[H3O+] = [OH-] neutral

There is also an internal relationship between the concentrations of H3O+ and OH-. They are not independent of one another. As one goes up, the other goes down. They cannot both go up because the higher concentrations of H3O+ and OH- would react with one another to make water molecules. That is a consequence of the reversibility of the self-ionization reaction of water. (2H2O dblarrow.gif (852 bytes) H3O+ + OH-)

Let's use the self-ionization of pure water as our starting point. The concentrations of both H3O+ and OH- are 1.0 x 10-7 M.  (For you notes, write these and the following values down in the table in exercise 24.)

If the concentration of H3O+ is doubled, the concentration of OH- willl be halved. If the concentration of H3O+ is halved, the concentration of OH- willl be doubled.

 
[H3O+] [OH-]
1.0 x 10-7 M
2.0 x 10-7 M
0.5 x 10-7 M 		1.0 x 10-7 M
0.5 x 10-7 M
2.0 x 10-7 M
If the concentration of H3O+ goes up by a factor of 10, to become 1.0 x 10-6 M, then the concentration of the OH- goes down by a factor of 10 become 1.0 x 10-8 M. If the concentration of H3O+ goes up by another factor of 10 to become 1.0 x 10-5 M, then the concentration of OH- goes down by another factor of 10 to become 1.0 x 10-9 M.

 
[H3O+] [OH-]
1.0 x 10-7 M
2.0 x 10-7 M
0.5 x 10-7 M
1.0 x 10-6 M
1.0 x 10-5 M 		1.0 x 10-7 M
0.5 x 10-7 M
2.0 x 10-7 M
1.0 x 10-8 M
1.0 x 10-9 M
This same pattern holds if the concentration of OH- is increased. Let's start again with neutral water. The concentrations of H3O+ and OH- are both 1.0 x 10-7 M. If the concentration of OH- is increased by a factor of 10 to become 1.0 x 10-6 M, then the concentration of H3O+ goes down by a factor of 10 to become 1.0 x 10-8 M. If the concentration of OH- is increased to 1.0 x 10-5 M, the concentration of H3O+ decreases to 1.0 x 10-9 M.
[H3O+] [OH-]
1.0 x 10-7 M
1.0 x 10-8 M
1.0 x 10-9 M 		1.0 x 10-7 M
1.0 x 10-6 M
1.0 x 10-5 M

 

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