Benzoic acid is titrated in a solvent mixture of 60 % ethanol and 40 % water by volume
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Benzoic acid is titrated in a solvent mixture of 60 % ethanol and 40 % water by volume. Solvent effects raise the acid dissociation constant to Ka=1.0×10−4 under these conditions. A 25.0 mL sample of 0.050 M benzoic acid is titrated with 0.100 M KOH prepared in the same solvent mixture at 25 °C. Indicator properties in the mixed solvent are summarized.
Indicator
pKa (mixed solvent)
Color-change interval (pH)
Thymol blue
2.4
1.6 – 3.2
Bromothymol blue
7.3
6.2 – 7.8
Phenolphthalein
9.7
8.6 – 10.4
Methyl orange, whose pKa remains 3.7 (interval 3.1 – 4.4), is also available.
(a) Calculate the volume of KOH required to reach the equivalence point.
(b) Determine the pH at the equivalence point, taking into account the solvent-modified Ka value.
(c) Using the indicator data, recommend the best choice for detecting the endpoint and justify your selection quantitatively.
(d) Suppose methyl orange is mistakenly used. Explain qualitatively whether the endpoint volume would be recorded too early, too late, or remain essentially correct, and discuss how this error would affect the calculated concentration of benzoic acid.