A regional water authority is investigating acidic runoff entering a river downstream of an old mine site. The authority also consults a local Aboriginal Ranger group, who describe a long-used practice of adding crushed shell and ash from specific plant material to small water samples and observing changes, to judge whether water is “sour” and needs treatment before use.
Two analysis approaches are trialled on the same day:
• Industrial-style titration (laboratory): A 25.00 mL sample of river water is titrated with 0.0100 mol L\(^{-1}\) NaOH. A calibrated pH probe is used to log pH after each addition.
• Field screening (on Country): Rangers add a measured mass of crushed shell (mostly CaCO\(_3\)) to 100.0 mL of the same river water in a jar, shake for 2 minutes, and then measure pH with a handheld digital probe.
The pH probe used in both settings is calibrated at 25\(^\circ\)C using pH 4.00 and pH 7.00 buffers. During fieldwork, the probe is used at 15\(^\circ\)C.
The titration data (selected points) are shown below.
| Volume of 0.0100 mol L\(^{-1}\) NaOH added (mL) | pH |
|---|---|
| 0.00 | 3.20 |
| 10.00 | 3.55 |
| 20.00 | 3.95 |
| 24.00 | 4.35 |
| 25.00 | 6.10 |
| 26.00 | 9.80 |
| 30.00 | 11.20 |
Assume the acidity in the river water behaves as if it is due to a single monoprotic strong acid for calculation purposes.
Explain how a digital pH probe improves the reliability of detecting the equivalence point in this titration compared with using a colour indicator alone. In your answer, refer to the given data pattern and to one limitation or source of error that still applies when using probes.
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Create Free Account Log inThis is a free HSC HSC Chemistry practice question worth 4 marks, testing your understanding of Explore acid/base analysis techniques that are applied. It falls under Quantitative Analysis in Module 6: Module 6: Acid/Base Reactions. Submit your answer above to receive instant AI-powered marking and personalised feedback.
Students analyse how and why the definitions of both an acid and a base have changed over time, and how the current definitions characterise the many chemical reactions of acids. Acids react in particular ways to a variety of substances. These reactions follow a pattern that students identify and explore in detail. Acids and bases, and their reactions, are used extensively in everyday life and in the human body. The chemistry of acids and bases contributes to industrial contexts and the environment. Therefore, it is essential that the degree of acidity in these situations is continually monitored. By investigating the qualitative and quantitative properties of acids and bases, students learn to appreciate the importance of factors such as pH and indicators.
Inquiry question: How are solutions of acids and bases analysed?
explore acid/base analysis techniques that are applied: - in industries - by Aboriginal and Torres Strait Islander Peoples - using digital probes and instruments
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