Chapter 11

\(0.5850 \mathrm{g}\) of NaCl were dissolved in \(100.0 \mathrm{cm}^{3}\) of water. \(10.0 \mathrm{cm}^{3}\) of this solution were made up with water to \(250.0 \mathrm{cm}^{3}\). Calculate the concentration of the resulting solution in: (a) moldm \(^{-3}\) (b) \(\left.\mathrm{molm}^{-3} ;(\mathrm{c}) \mathrm{mg} \mathrm{dm}^{-3} ;(\mathrm{d}) \text { ppm (by mass). (Section } 11.1\right)\)

In preparing a standard solution, you forget to zero the balance so that it reads \(0.10 \mathrm{g}\) too high. What kind of error is this? What effect would this error have on an analysis if the standard solution was used to prepare a calibration graph by dilution? (Section 11.1 )

A laboratory operates two methods for determining the concentration of lead in water. To check the method, a Certified Reference Material containing a known Pb concentration of \(20.00 \mu g d m^{-3}\) is analysed ten times using each of the two methods. The first method gives a mean concentration of \(21.9 \mu \mathrm{g} \mathrm{dm}^{-3}\) with a standard deviation of \(0.5 \mu \mathrm{g} \mathrm{dm}^{-3} .\) The second method gives a mean and standard deviation of \(19.4 \mu \mathrm{g} \mathrm{dm}^{-3}\) and \(2.1 \mu \mathrm{g} \mathrm{dm}^{-3},\) respectively. Comment on these results in terms of the accuracy and precision of the two analytical methods. (Section 11.1 )

An ion selective electrode is designed to measure the concentration of perchlorate ions \(\left(\mathrm{ClO}_{4}\right)\). The electrode was immersed in \(50.0 \mathrm{cm}^{3}\) of an unknown solution of perchlorate ions and registered a potential of \(358.7 \mathrm{mV}\) against a standard electrode. \(0.50 \mathrm{cm}^{3}\) of a solution of \(\mathrm{NaClO} 4(\mathrm{aq})\) of concentration \(0.1 \mathrm{moldm}^{-3}\) was then added and the potential changed to \(346.1 \mathrm{mV} .\) What was the concentration of \(\mathrm{ClO}_{4}\) -ions in the unknown solution? (Section 11.2 )

A solute has a partition coefficient of 28.7 between trichloromethane \(\left(\mathrm{CHCl}_{2}\right)\) and water. \(20 \mathrm{cm}^{3}\) of \(\mathrm{CHCl}_{3}\) were added to \(100 \mathrm{cm}^{3}\) of an aqueous solution of the solute. At equilibrium, the aqueous layer contained a concentration of \(0.005 \mathrm{moldm}^{-3} .\) What was the concentration of the organic layer? (Assume \(k=1\).) (Section 11.3 )

You are handed a bottle labelled 'xylene' (dimethylbenzene). A gas chromatogram gives three peaks corresponding to the \(1,2-,\) the \(1,3-,\) and the 1,4 -isomers with peak areas in the ratio of \(143.1: 9.5: 6.4,\) respectively. Assuming that the detector responds equally to each isomer, calculate the composition of the xylene. (Section 11.3 )

Suggest suitable forms of chromatography to investigate the following situations. (Section 11.3 ) (a) You are an analyst in a dairy. It is claimed that a consignment of milk has been contaminated with a toxic organochlorine pesticide at ppb levels. How would you identify and measure the concentration of the pesticide? (b) Your colleagues in a pharmaceutical company have developed a new high- yield route to a single enantiomer of a chiral target compound. You are asked to check the purity of the product and to identify any by-product impurities in it and measure their concentrations. How might you do this?

Four solutions of a dye were prepared in water. In a \(1.00 \mathrm{cm}\) cell, the percentage of light at a particular wavelength transmitted through each solution is given in the following table. $$\begin{array}{lllll}\hline \text { Concentration/moldm }^{3} & 0.004 & 0.010 & 0.020 & 0.040 \\\\\text { Percentage light transmitted } &79 & 56 & 32 & 10 \\\\\hline\end{array}$$ Confirm that these data obey the Beer-Lambert law and calculate the molar absorption coefficient, \(\varepsilon,\) at this wavelength. (Section \(11.4)\)

As part of an investigation into the harmful effects of petrol engine emissions, a \(10 \mathrm{g}\) sample of grass from a roadside verge was analysed for its lead content. The grass was burned to ash in oxygen to remove organic material and the inorganic residue was dissolved in \(20 \mathrm{cm}^{3}\) of dilute acid. Under certain conditions, this gave an absorbance of 0.72 on an atomic absorption spectrometer. A standard solution containing \(1.0 \mu \mathrm{gcm}^{-3}\) of lead was available. Aliquots of this were made up to \(50 \mathrm{cm}^{3}\) with dilute acid and the absorbances measured as shown in the table. $$\begin{array}{llllll}\hline \text { Volume of standard } & 5 & 10 & 15 & 20 & 25 \\\\\text { solution in } 50 \mathrm{cm}^{3} /\mathrm{cm}^{3} & & & & & \\\\\text { Absorbance } & 0.26 & 0.52 & 0.81 & 1.04 & 1.30 \\\\\hline\end{array}$$ Calculate the concentration of lead in the grass in parts per million (by mass). (Section 11.4 )

A sample of sodium sulfate is known to be contaminated with copper ions. Analysis of reference solutions containing copper ions at known concentrations gave the absorbances shown in the table. Under the same conditions, a solution made by dissolving \(1.00 \mathrm{g}\) of the sodium sulfate sample in \(1.00 \mathrm{dm}^{3}\) distilled water gave an absorbance of \(0.20 .\) What was the purity of the sodium sulfate, assuming that copper ions are the only impurity? (Section 11.4 ) $$\begin{array}{llllll}\hline \text { Concentration of } \mathrm{Cu}^{2+} / \mathrm{ppm} & 2.0 & 4.0 & 6.0 & 8.0 & 10.0 \\\\\text {Absorbance } & 0.06 & 0.12 & 0.18 & 0.24 & 0.30 \\\\\hline\end{array}$$

Tributyltin chloride, \(\mathrm{TBT}\), was commonly added to marine paints to prevent the growth of barnacles on the hulls of ships although it is now banned due to its toxicity. A common method for analysing TBT in marine sediments is graphite furnace atomic absorption spectroscopy (AAS). In a particular analysis, \(0.500 \mathrm{g}\) of a sediment was dissolved in \(10.0 \mathrm{cm}^{3}\) of acid and aspirated into a graphite furnace AAS. The sample gave an absorbance of 0.44 A standard solution of tin with an accurately known concentration of \(1.0 \mu \mathrm{g} \mathrm{dm}^{-3}(1.0 \mathrm{ppb})\) was available. Known volumes of this were added to \(1.00 \mathrm{dm}^{3}\) volumetric flasks and made up to the mark with pure water. Under identical conditions in the \(\mathrm{AAS}\), these solutions gave the following results. $$\begin{array}{rr}\hline \text { Volume of standard added/cm }^{3} & \text { Absorbance } \\\\\hline 5 & 0.13 \\\10 & 0.26 \\\15 & 0.39 \\\20& 0.52 \\ 25 & 0.65 \\\\\hline\end{array}$$ Find the concentration of \(\mathrm{TBT}\) in the sediment. (Section 11.5 )