A 100 mL sample of 0.075 M sulfurous acid, H₂SO₃ (a diprotic acid)
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 100 mL sample of 0.075 M sulfurous acid, H₂SO₃ (a diprotic acid), is titrated with 0.150 M NaOH at 25 °C. The acid dissociation constants are Ka1=1.0×10−2 and Ka2=6.2×10−8. A selection of indicators is available.
Indicator
pKa
Color-change interval (pH)
Methyl orange
3.7
3.1 – 4.4
Bromothymol blue
7.1
6.0 – 7.6
Phenolphthalein
9.3
8.2 – 10.0
(a) Write the net ionic equation for the reaction that occurs at the first equivalence point and the reaction that occurs at the second equivalence point.
(b) Calculate the pH at the first equivalence point.
(c) Calculate the pH at the second equivalence point.
(d) For each equivalence point, identify the indicator from the table that would provide the most accurate endpoint detection and justify both choices quantitatively.