CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 25.0 mL sample of 0.100 M acetic acid, CH₃COOH, is titrated with 0.100 M NaOH at 25 °C. The acid dissociation constant of acetic acid is Ka=1.8×10−5. A list of potential indicators is provided.
Indicator
pKa
Color-change interval (pH)
Thymol blue
2.0
1.2 – 2.8
Methyl orange
3.7
3.1 – 4.4
Bromothymol blue
7.1
6.0 – 7.6
Phenolphthalein
9.3
8.2 – 10.0
(a) Calculate the volume of NaOH that must be added to reach the equivalence point.
(b) Determine the pH at the equivalence point.
(c) From the indicators listed, identify (i) the best and (ii) the second-best indicator for this titration. For each choice, justify your reasoning with quantitative evidence.
(d) Predict qualitatively how the pH at the equivalence point and your indicator selection would change if the initial acetic-acid concentration were decreased to 0.050 M while all other conditions remained the same.