An experiment was conducted to determine the lead content of 10.00 g sample of shark meat. The meat was  dissolved in 20.00mL of 1.00 M hydrochloric acid (HCl) to which 30mL  of water was added to dilute the sample.
The resulting solution was filtered and 10.00 mL of the filtrate was mixed with 20.00 mL of a solution that gave a coloured solution as it reacted with the lead ions. In the final mixture, lead from the shark meat was present as Pb²⁺(aq). Using a UV-visible colorimeter, the absorbance of this final solution was measured using a 2.00 mL sample. The absorbance was 0.110.

Absorbances of the following standard solutions were also obtained.

Using the above data and the graph paper provided, construct a calibration curve.

• What was the concentration percentage (m/v) of lead ions in the final mixture?

Solution................................... approximately 0.0124%(m/v).....................................................................................

• What was the concentration percentage (m/m) of lead in the original sample?

Solution.................................................................................................................................................................
................................................................0.186%(m/m)................................................................................................

• What is the concentration of lead ions in the original sample in ppm?

Solution.................................................................................................................................................................       
...............................................................1860 ppm.................................................................................................

• Determine the mole of lead ions present in the original sample of shark meat.

..........................................................0.0186g/207.2 = 8.98 X 10^-5..........................................................................
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• What was the molarity of lead in the solution made by dissolving the shark meat  in 20.00 mL of 1.00 M HCl (hydrochloric acid) and 30 mL of water?                                                                       

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.................................................8.98 X 10^-5 / 0.05 = 0.0018M............................................................

• a) Using a UV-visible colorimeter, as outlined above, is it necessary to:
• know the molarity of the HCl used to dissolve the shark meat? Explain

No! The molarity is not used in any calculation. It is only the volume that is required as the analytical technique used gives the concentration value.

• know the wavelength of light used? Explain

Yes. In order to measure the absorbance we have to carefully select a wavelength that is absorbed by the solution and in particular the lead ions.

• When is it necessary to dilute the sample before inserting it in the spectrometer?           

1) When the concentration of the sample is too high.

2) When the solution is opaque and no light can pass through to be measured.

• Why are calibration curves necessary for each investigation?

.Calibration curves give a relationship between the concentration and the absorbance. This relationship differs for each equipment used and may differ as a result of other external factors such concentration of carbon dioxide in the laboratory as with ir-spectroscopy.

• In quantitative analysis which is more accurate, atomic absorption, UV-visible spectroscopy or infrared spectroscopy. Explain.

Atomic absorption uses the unique wavelength that is absorbed by a particular element. Hence it is the most accurate.

• Which of the following techniques uses a calibration curve?
• Atomic absorption spectroscopy
• Uv-visible spectroscopy
• Mass spectroscopy
• Infrared spectroscopy

Atomic absorption, uv-visible, mass spectroscopy
Extra reading

• Which of the following techniques uses a reference cell?
• Atomic absorption spectroscopy
• Uv-visible spectroscopy
• Mass spectroscopy
• Infrared spectroscopy

Ir-spectroscopy and uv-visible spectroscopy.

• What is the purpose of a reference cell?

Quite often the container in which the sample is held can absorb the radiation as can the solvent. It is important to take this absorbance into account and eliminate it from the measurements. A reference cell allows us to do this.