Why, when deriving the formula for calculating the equilibrium of an acidic buffer solution, is it considered that [An “] = C (salt)?

When deriving the formula for calculating the equilibrium of an acidic buffer solution, it is assumed that [An ~] = Csalt, since the presence of an anion reduces the degree of dissociation of a weak acid and the contribution of dissociated acid molecules to the total concentration of an anion is insignificant.

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