In a titration of an acetic acid (CH3COOH) solution of unknown concentration, a
ID: 964886 • Letter: I
Question
In a titration of an acetic acid (CH3COOH) solution of unknown concentration, a 28.18 ml sample of the acid is required 28.73ml of a 0.2110 mol/L KOH solution to reach the equivalence point.
a) Determine the initial concentration of CH3COOH
b) What ions are present in significant concentrations in the solution at the equivalence point? Support answer with the appropriate balanced chemical equation
c) What is the concentration of the conjugate base at the equivalence point?
d) What is the percent dissociation, given an initial pH of 2.82?
e) Determine the pH of the solution at the equivalence point.
Explanation / Answer
a ) at equivalent point
millimoles of acid = millimoles of base
28.18 x C = 28.73 x 0.2110
C = 0.215 M
initial concentration of CH3COOH = 0.215 M
b)
CH3COOH + KOH ------------------> CH3COOK + H2O
at equivalence point only salt is present . the salt is CH3COOK . ion present CH3COO- and Na+
c) concentration of conjugate base = millimoles / total volume
= 28.73 x 0.2110 / (28.18 + 28.73)
= 0.1065 M
d) pH = 2.82
[H+] = 10^-2.82 = 1.51 x 10^-3 M = x
percent dissociation = ( x^2 / C ) x100
= (1.51 x 10^-3)^2 / 0.215 ) x 100
= 0.00107%
e) at equivalence point
C = 0.1056 M
pH = 7 + 1/2 [pKa + log C]
pH = 7 + 1/2 [4.74 + log 0.1065]
pH = 8.88
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