In a titration with a strong acid and weak base, where is the equivalence point?

Feb 17, 2016

When a strong acid is titrated by a weak base, the equivalence point will be less than 7 ($p H < 7$).

Explanation:

When a strong acid is titrated by a weak base, the equivalence point will be less than 7 ($p H < 7$).

At the equivalence point, the acid is completely consumed and the conjugate base of the strong acid will have no affinity to ${H}^{+}$ and therefore, the major species is the conjugate acid of the base which will make the solution slightly acidic and therefore, the pH will be less than 7.

For example, if we are titrating $H C l$ by $N {H}_{3}$, the reaction at the equivalence point will be:

${H}^{+} \left(a q\right) + N {H}_{3} \left(a q\right) \to N {H}_{4}^{+} \left(a q\right)$

And therefore, the conjugate acid $N {H}_{4}^{+}$ will dissociate in water according to the following reaction:

$N {H}_{4}^{+} \left(a q\right) \to {H}^{+} \left(a q\right) + N {H}_{3} \left(a q\right)$

the $p H$ will be determined by the concentration of ${H}^{+}$, ($\left[{H}^{+}\right]$).

$p H = - \log \left[{H}^{+}\right]$.

Here is a video that discusses the titration of a weak acid by a strong base, which is somehow similar in terms of calculation of a strong acid by a weak base:
Acid - Base Equilibria | Weak Acid - Strong Base Titration.