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Transition metals (A-level only)

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Question 6

The copper content of a dietary copper supplement tablet can be determined using colorimetric analysis.

Method:

  • A tablet is dissolved in concentrated nitric acid.
  • The mixture is neutralized and excess aqueous ammonia is added to convert all copper into the deep-blue complex ion, [Cu(NH3)4(H2O)2]2+[\text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+}[Cu(NH3​)4​(H2​O)2​]2+.
  • The solution is made up to 100 cm3100\text{ cm}^3100 cm3 with distilled water in a volumetric flask.
  • This complex absorbs light strongly at a wavelength of 620 nm620\text{ nm}620 nm.
  • The absorbance of the solution is measured at 620 nm620\text{ nm}620 nm and a calibration graph is used to find the concentration of the copper complex.

Questions:

1.

Calculate the energy, in J\text{J}J, gained by each excited electron in the absorption of light at 620 nm620\text{ nm}620 nm. (Speed of light, c=3.00×108 m s−1c = 3.00 \times 10^8\text{ m s}^{-1}c=3.00×108 m s−1; Planck constant, h=6.63×10−34 J sh = 6.63 \times 10^{-34}\text{ J s}h=6.63×10−34 J s)

[3]
2.

The concentration of the copper complex in the 100 cm3100\text{ cm}^3100 cm3 solution is determined to be 3.75×10−3 mol dm−33.75 \times 10^{-3}\text{ mol dm}^{-3}3.75×10−3 mol dm−3. Calculate the mass, in mg\text{mg}mg, of copper in the tablet. (Relative atomic mass, Ar(Cu)=63.5A_r(\text{Cu}) = 63.5Ar​(Cu)=63.5)

[3]

Transition metals (A-level only) Questions

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