If the pH is 3, what is the hydronium ion concentration?

ooo my fav qs lol

pH = - log [H30+] remember this. You will need a calculator and understand logarithms (helps to do A level maths )   **Note: ILL use   [H+] to mean the hydronium ion.


If the pH = 3, then, using the formula:

     -3 = log [H+]    so   [H+] = 10^-3 = 0.001 M

 when pH = 7:

    -7 = log [H+]   so  [H+] = 10^-7 = 0.0000001 M

see the trick?

     Now for the hydroxide ion -

You need to remember Kw = 10^-14 = [H+][OH-]

and if the pH is 3, then [H+] = 10^-3.  

So 10^-14/10^-3 =   [OH-] = 10^-11 M.

Similarly:
With pH 8,  
   -8 = log [H+]    so [H+] = 10^-8 M

10^-14/10^-8 = 10^-6 M.

And remember  "M" is molarity, that's the conc.  

(1) [H3O+] = 1x10^-3
(2) 1x10^-7
(3)1x10^--14
(4) 1x10^-11
(5) 1x10^-6