Chemistry
Chemistry
Problem
A chemist titrates a 50.0 mL sample of 0.100 M hydrochloric acid with 0.100 M sodium hydroxide at 25 °C. A selection of acid–base indicators is provided.| 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 |
Intuitive explanation
This is a strong acid–strong base titration, where the equivalence point occurs at pH 7, but the key is choosing an indicator whose color-change range brackets that endpoint. Since the acid and base have equal molarity and you're using equal volumes, they'll neutralize completely at the equivalence point. You need to pick the indicator whose pKa (and thus its color-change interval) overlaps with where the pH actually jumps during the titration—not necessarily at pH 7, but in the steep region where a tiny drop of base causes a huge pH shift. Think about which indicator's transition range will catch that sharp vertical leap on the titration curve.
Intuitive explanation
This is a strong acid–strong base titration, where the equivalence point occurs at pH 7, but the key is choosing an indicator whose color-change range brackets that endpoint. Since the acid and base have equal molarity and you're using equal volumes, they'll neutralize completely at the equivalence point. You need to pick the indicator whose pKa (and thus its color-change interval) overlaps with where the pH actually jumps during the titration—not necessarily at pH 7, but in the steep region where a tiny drop of base causes a huge pH shift. Think about which indicator's transition range will catch that sharp vertical leap on the titration curve.