# How do you determine pH at an equivalence point?

Mar 18, 2014

When all of a weak acid has been neutralized by strong base, the solution is essentially equivalent to a solution of the conjugate base of the weak acid.

For example, if a 0.2 M solution of acetic acid is titrated to the equivalence point by adding an equal volume of 0.2 M NaOH, the resulting solution is exactly the same as if you had prepared a 0.1 M solution of sodium acetate.

The pH of 0.1 M sodium acetate is calculated as follows:

${K}_{b}$ = $5.56 x {10}^{- 10}$ = $\frac{\left[O {H}^{-}\right] \left[H A\right]}{\left[{A}^{-}\right]}$ = ${x}^{2} / \left(0.1 - x\right)$ &approx; ${x}^{2} / 0.1$
x = ${\left(0.1 {K}_{b}\right)}^{\frac{1}{2}}$ = $7.46 x {10}^{- 6}$ = [$O {H}^{-}$]
pOH = -log($7.46 x {10}^{- 6}$) = 5.13
pH = 14 - pOH = 8.87