A water-quality officer investigates chloride contamination in a wetland downstream of a de-icing salt storage area. A 250.0 mL sample of filtered wetland water is collected.
Two different analytical methods are trialled on separate aliquots of this sample.
Method 1: Gravimetric analysis (precipitation and weighing)
A 25.00 mL aliquot is treated with excess \(\text{AgNO}_3(aq)\) to precipitate silver chloride. The precipitate is filtered, washed, dried to constant mass, and weighed.
Mass data:
- Mass of clean, dry filter crucible: 32.684 g
- Mass of crucible + dry precipitate: 33.041 g
Assume the precipitate is pure \(\text{AgCl}(s)\).
Method 2: Precipitation titration (Mohr method)
A 10.00 mL aliquot is titrated with \(0.02000\ \text{mol L}^{-1}\) \(\text{AgNO}_3(aq)\) using \(\text{K}_2\text{CrO}_4(aq)\) as indicator. The endpoint is the first permanent red-brown colour due to \(\text{Ag}_2\text{CrO}_4(s)\).
Titration results:
- Trial 1: 12.41 mL
- Trial 2: 12.36 mL
- Trial 3: 12.40 mL
Relevant molar masses:
- \(M(\text{AgCl}) = 143.32\ \text{g mol}^{-1}\)
Assume \(\text{Ag}^+(aq)\) reacts \(1{:}1\) with \(\text{Cl}^-(aq)\) to form \(\text{AgCl}(s)\).
The two methods give substantially different chloride concentrations. Identify one plausible experimental issue specific to the gravimetric method that would cause its calculated \([\text{Cl}^-]\) to be too high, and explain how it leads to a higher result.
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Create Free Account Log inThis is a free HSC HSC Chemistry practice question worth 3 marks, testing your understanding of Conduct investigations and/or process data involving. It falls under Analysis of Inorganic Substances in Module 8: Module 8: Applying Chemical Ideas. Submit your answer above to receive instant AI-powered marking and personalised feedback.
The identification and analysis of chemicals is of immense importance in scientific research, medicine, environmental management, quality control, mining and many other fields. Students investigate a range of methods used to identify and measure quantities of chemicals. They investigate and process data involving the identification and quantification of ions present in aqueous solutions. This is particularly important because of the impact of adverse water quality on the environment. Students deduce or confirm the structure and identity of organic compounds by interpreting data from qualitative tests of chemical reactivity and determining structural information using proton and carbon-13 nuclear magnetic resonance (NMR) spectroscopy.
Inquiry question: How are the ions present in the environment identified and measured?
conduct investigations and/or process data involving: - gravimetric analysis - precipitation titrations
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