General Chemistry

There is a formula linking the values of pH and pOH which allows determining the pH or pOH based on one another.

It is derived from the expression of water ionization, Kw = [H+][OH].

To get the pH and pOH correlation, we take a negative log of both sides of the equation:

 

-log [H+] + (- log [OH]) = – log 10-14

 

Notice that the terms on the left represent the pH and pOH of the solution (-log [H+] = pH, – log [OH] = pOH). Therefore, we can write that:

 

pH + pOH = 14

 

Let’s see how this can be used to calculate, for example, the pH from pOH.

 

For example, determine the pH of a 0.850 M NaOH.

The strategy here is to calculate the pOH and then, using the pH + pOH = 14 relationship, determine the pH.

NaOH is a strong base, so [OH] = [NaOH], and therefore, the pOH is:

 

pOH = -log 0.850 = 0.0706

 

The pH then is equal to:

 

pH = 14 – pOH = 14 – 0.0706 = 13.9

 

As expected, the pH is very high because it is a solution of a strong base.

 

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