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## pH calculation questions » finding pKa of weak acid

0.1 M solution of weak acid has pH=4.0. Calculate pK_{a}.

If pH=4.0 then [H^{+}]=10^{-4} - that means that the dissociation fraction is about

10^{-4}/0.1×100%=0.1% - well below 5%. That in turn means that pH of the solution is described by the simplified formula 8.13:

All we have to do is rearrange to get the equation for K_{a}:

Plugging numbers in we get K_{a}=(10^{-4})^{2}/0.1=10^{-7}, so **pK _{a}=7**.

Our pH calculator can help us check this result - and indeed, entering 0.1 as concentration and 7 as pK_{a} we get pH=4.0 which confirms our calculations.