Question

How do you determine pH at an equivalence point?

Answer 1

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` = `5.56x10^(-10)` = `([OH^-][HA])/([A^-])` = `x^2/(0.1-x)` ≈ `x^2/0.1`
x = `(0.1 K_b)^(1/2)` = `7.46x10^(-6)` = [`OH^-`]
pOH = -log(`7.46x10^(-6)`) = 5.13
pH = 14 - pOH = 8.87