BEHS Chemistry

The Math of PH


PH is defined as the NEGATIVE log of the hydrogen ion concentration


PH = -log [H +]


Because H + is always in equilibrium with water and INVERSLY proportional to OH - concentration (as one increases the other decreases).



HOH <---> H + + OH -


We can also have the "POWER of the hydroxide ion" value, or pOH:


POH = -log [ OH -]


Also remember the ion product constant for water:


[H +] x [ OH -] = 1x10 -14 moles/liter 2


With some mathematical manipulation this also means that:


PH + pOH = 14


With all this information, you can solve problems involving pH:


1. What is the pH of a solution with a [H +] of 1 x 10 -8?


2. What is the [H +] of a solution with a pH of 3?


3. What is the pOH of a solution with a pH of 5?


4. What is the [ OH -] of a solution with a pH of 4?


5. If you wanted to change the pH of a solution from 3 to 5, how many times would you dilute it?


Contact: Tom Davidson, Box Elder High School 380 So. 600 w. (435-734-4840) Email: