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## pH calculation questions » pH of weak acid

Calculate pH of 0.001M solution of benzoic acid, pK_{a}=4.19.

We will start with the simplified formula 8.13.

From formula we get [H^{+}]=2.54×10^{-4} M and **pH=3.60**. Is it correct? To be sure we have to check if the 5% rule is obeyed, that is if the dissociation fraction is less than 5%.

Dissociated acid concentration is the same as [H^{+}], thus dissociation fraction is 2.54×10^{-4}/0.001 - above 25%. We can't use simplified formula 8.13! We have to use more precise equation 8.10:

This time we get **pH=3.65**, exactly the same result as from our pH calculator.