Dissociation of Water and pH

 

            Water has a tendency to dissociate (break up) into ions when in solution.  The result is the equation I talked about:

           

            H2O + H2O ßà H3O+ + OH-

 

We can actually measure the concentration of these 2 ions.  When measured both the concentration of H3O+ and OH- are 0.0000001 mol/L or 1 x 10-7mol/L.  Check out this website for an animation of this process:

http://www.scidiv.bcc.ctc.edu/wv/acid_base/dissociation_of_water.html

 

Now, if we add an acid to pure water we will increase the concentration of the H3O+ ion.  If we add a base, we will increase the OH- concentration and decrease the H3O+ concentration.

 

An acid increases the H3O+ concentration by donating a proton (H+) when dissolved in water:

 

                        HCl  +  H2O à H3O+ + Cl-

 

So, an acid again, increases the H3O+ concentration.

 

A base is a proton acceptor when dissolved in water, which increases the OH- concentration:

 


                        NH3 + H2O ßà (NH4)+ + OH-

 

So in these cases we have changed the H3O+ concentration from 1 x 10-7 mol/L to another value.  With the acid we have increased the H3O+ concentration and for the base we have decreased it.  So our concentrations would look like this:

           

1

1 x 100

High H3O+ concentration

Low OH- concentration

   

 0.1

1 x 10-1

 

 

 

0.01

1 x 10-2

 

 

 

0.001

1 x 10-3

 

 

 

0.0001

1 x 10-4

 

 

 

0.00001

1 x 10-5

 

 

 

0.000001

1 x 10-6

 

 

 

0.0000001

1 x 10-7

Equal concentration of H3O+ and OH-

 

0.00000001

1 x 10-8

 

 

 

0.000000001

1 x 10-9

 

 

 

0.0000000001

1 x 10-10

 

 

 

0.00000000001

1 x 10-11

 

 

 

0.000000000001

1 x 10-12

 

 

 

0.0000000000001

1 x 10-13

 

 

 

0.00000000000001

1 x 10-14

Low H3O+ concentration

High OH- concentration

 

 

Now, using the values in that table would be pretty cumbersome, so the pH scale allows us to express these values in much simpler terms.

 

The term pH means to take the –log10 [H+ concentration] or [H3O+ concentration]. 

 

The pH scale runs from 0-14 with 0 being most acidic and 14 being most basic and 7 being neutral.

 

Each change in 1 on the pH scale (Ex. 1 to 2 or 8 to 7) indicates a tenfold change in H3O+ concentration.

 

The pH value plus the pOH value always equals 14.

 

So, what are the pH values for the concentrations in the table above?  Remember use this formula:

pH =  -log10[H3O+mol/L]

 

Remember (mol/L) simply tells you the amount of H3O+ ions per liter (L)

 

For more info and to check your answers go back to the above website and click on the à at the top of the page.  You’ll have to click twice to check your answers.