Mr Cole Chemistry

Colourimetry Questions

  • Questions
  • Answers
  1. A piece of copper was analysed to determine the purity using the following steps.
    • A 4.00g sample of the copper was dissolved in concentrated sulfuric acid to make an aqueous copper (II) solution.
    • The solution had excess concentrated ammonia added to it. The reaction that occurred formed a blue precipitate, which then redissolved to form a dark blue solution.
    • The solution was topped up to 250cm3 in a volumetric flask.
    • The resulting solution was poured into a 1cm3 cuvette of known dimensions.
    • An orange filter was picked for the colorimeter.
    • The cuvette containing the copper (II) solution was placed in the colorimeter and a reading was obtained.
    • A calibration curve was used to determine the concentration of the copper ions.
    • It was found to have an absorbance of 9.0.
      1. State the purpose of adding concentrated ammonia.
      2. State why the cuvette has to have specific dimensions.
      3. Suggest why an orange filter is used for the colorimetry.
      4. Explain how a calibration curve is made and used to determine the concentration of the copper ions.
      5. Using the table of results for the calibration curve below, draw a graph for the curve and use it to determine the percentage purity of the copper.
  2. A separate colorimetry experiment is set up to determine the formula of a copper ammonia complex. A calibration curve is set up by adding different volumes of 0.2 moldm-3 ammonia solution to 35cm3 of 0.2 moldm-3 copper sulfate solution. The results are in the table below.
    1. Draw a calibration curve on the graph paper below.`
    2. A sample of copper ammonia solution was analysed and found that the absorbance was 141.
      1. Calculate the moles of copper ions in the solution.
      2. Calculate the moles of ammonia in the solution.
      3. Calculate the ratio of the copper ions to ammonia in the solution.
      4. Suggest a structure for the complex by drawing it.
    3. A different sample of copper ammonia solution was analysed and found that the absorbance was 86.
      1. Calculate the moles of ammonia in the solution.
      2. Calculate the ratio of the copper ions to ammonia in the solution.
      3. Suggest a structure for the complex by drawing it.
  1. A piece of copper was analysed to determine the purity using the following steps.
    • A 4.00g sample of the copper was dissolved in concentrated sulfuric acid to make an aqueous copper (II) solution.
    • The solution had excess concentrated ammonia added to it. The reaction that occurred formed a blue precipitate, which then redissolved to form a dark blue solution.
    • The solution was topped up to 250cm3 in a volumetric flask.
    • The resulting solution was poured into a 1cm3 cuvette of known dimensions.
    • An orange filter was picked for the colorimeter.
    • The cuvette containing the copper (II) solution was placed in the colorimeter and a reading was obtained.
    • A calibration curve was used to determine the concentration of the copper ions.
    • It was found to have an absorbance of 9.0.
      1. State the purpose of adding concentrated ammonia.
        To form the complex [Cu(NH3)4(H2O)2]2+, which gives a deeper colour that is more easily picked up by the colorimeter.
      2. State why the cuvette has to have specific dimensions.
        The distance the light travels through the cuvette affects the amount of light absorbed and therefore has to be the same every time.
      3. Suggest why an orange filter is used for the colorimetry.
        The [Cu(NH3)4(H2O)2]2+ complex is blue and therefore only absorbs orange light.
      4. Explain how a calibration curve is made and used to determine the concentration of the copper ions.
        measure absorbance for (a range of) known concentrations
        plot graph absorbance v concentration
        read value of concentration for the measured absorbance from this graph
      5. Using the table of results for the calibration curve below, draw a graph for the curve and use it to determine the percentage purity of the copper.


        Absorbance: 9
        Concentration: 0.18 mol dm-3
        Moles in 250cm3: 0.18 * 0.25 = 0.045
        Mass of copper: 0.045 x 63.5 = 2.86g
        Purity: 71.4%
  2. A separate colorimetry experiment is set up to determine the formula of a copper ammonia complex. A calibration curve is set up by adding different volumes of 0.2 moldm-3 ammonia solution to 35cm3 of 0.2 moldm-3 copper sulfate solution. The results are in the table below.
    1. Draw a calibration curve on the graph paper below.`
    2. A sample of copper ammonia solution was analysed and found that the absorbance was 141.
      1. Calculate the moles of copper ions in the solution.
        Moles = concentration x volume
        0.2 x 0.035 = 0.007 moles
      2. Calculate the moles of ammonia in the solution.
        Absorbance: 141
        Volume of NH3: 140 – 150
        Moles: 0.2 x 0.14 = 0.028
      3. Calculate the ratio of the copper ions to ammonia in the solution.
        0.007: 0.028
        1: 4 Cu2+ : NH3
      4. Suggest a structure for the complex by drawing it.
    3. A different sample of copper ammonia solution was analysed and found that the absorbance was 86.
      1. Calculate the moles of ammonia in the solution.
        Absorbance: 86
        Volume of NH3: 69 (+/- 2)
        Moles: 0.2 x 0.069 = 0.0138
      2. Calculate the ratio of the copper ions to ammonia in the solution.
        0.007 : 0.0138 Cu2+ : NH3
        1: 1.97 = 1:2
      3. Suggest a structure for the complex by drawing it.

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