2 assumptions are made when calculating the concentration of weak acids, those being:
the dissociation of the weak acid is so small it is neglible, so the concentration of the weak acid stays the same.
the little that does dissociate is the same concentration of the h+ ions that it contributes to the solution. So the concentration of the salt ion is = the H+ ion concentration (no H+ donated to solution from water)
To work out the pH, you use the formula Ka = [H+][A-]/[HA]. Ka is the acid dissociation constant. A is the other part of the acid. for example, in ethanoic acid (CH3COOH) the A would be CH3COO-.
It is also important to note that [] signifies "the concentration of".
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