[eeyore1228]

A solution is prepared by adding some unknown amount of sodium hydroxide to a volumetric flsk and diluting the solution to 250.0 mL.  You measure the pH using a pH meter to be 12.4.  What is the hydronium ion concentration?
What is the hydroxide concentration?

I do not understand how to convert a given pH into the ion concentration.  I know that the OH- and the H3O+ together needs to be 14.  

[hmx9123]

Remember that pH is equal to the negative 10-base log of the hydrogen ion concentration.

pH = - log [H⁺ ]

[Borek]

Look here for definition and some very basic stuff about pH:

http://www.chembuddy.com/?left=pH-calculation&right=introduction-acid-base-equilibrium

[arnyk]

Alright, so pH = -log [H3O+] is how to figure out pH from the [].

To figure it out the other way around from [H3O+] to pH you would go:

[H3O+] = 10^-pH

The hydronium ion concentration is equal to 10 to the power of the negative pH.

Example:  What is the [H3O+] in a solution with pH = 2.

[H3O+] = 10^-pH
            = 10^(-2)
             = 0.01 M