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## pH calculation questions » pH of weak acid

Calculate pH of 1M solution of nitrous acid, pK_{a}=3.37.

Let's try first with the simplified formula 8.13.

If we put concentration and K_{a} values into formula we get [H^{+}]=0.021 M and **pH=1.69**. 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^{+}] - if we assume we can neglect water dissociation. It is obvious that we can, as 0.021 M is five orders of magnitude larger than the concentration of H^{+} from the water. So dissociation fraction is 0.021/1 - a little bit over 2%, much less than 5%, so the 5% rule is obeyed.

As additional confirmation our pH calculator shows exactly the same result.