This is about a titration. The base titrant used is sodium hydroxide.
1 Answer
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You do not assume 100% ionization of acetic acid during titration. Acetic acid is a weak acid. Look at this equilibrium
$\ce{CH3COOH <=> H+ + CH3COO-}$
The base, $\ce{NaOH}$, keeps consuming $\ce{H+}$ during a titration, as a result acetic acids keeps on providing $\ce{H+}$ to maintain the equilibrium in water until all the acetic acid is converted into acetate ion.