How do you determine pH at an equivalence point?

1 Answer
Chuck W.
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# = #5.56x10^(-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

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