How do you determine pH at an equivalence point?

1 Answer
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_bKb = 5.56x10^(-10)5.56x1010 = ([OH^-][HA])/([A^-])[OH][HA][A] = x^2/(0.1-x)x20.1xx^2/0.1x20.1
x = (0.1 K_b)^(1/2)(0.1Kb)12 = 7.46x10^(-6)7.46x106 = [OH^-OH]
pOH = -log(7.46x10^(-6)7.46x106) = 5.13
pH = 14 - pOH = 8.87