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5 October, 20:18

Molar absorptivity data for the cobalt and nickel complexes with 2,3-quinoxalinedithiol are = 36400 and = 5520 at 510 nm and = 1240 and = 17500 at 656 nm. A 0.411-g sample was dissolved and diluted to 50.0 mL. A 25.0-mL aliquot was treated to eliminate interferences; after addition of 2,3-quinoxalinedithiol, the volume was adjusted to 50.0 mL. This solution had an absorbance of 0.483 at 510 nm and 0.367 at 656 nm in a 1.00-cm cell. Calculate the concentration in parts per million of cobalt and nickel in the sample.

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  1. 5 October, 20:45
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    Check the solution

    Explanation:

    Absorbance of solution = Absorbance due to Co + absorbance due to Ni

    Absorbance = molar absorptivity x concentration x path length

    with path length = 1 cm

    molar absorptivity at 510 nm,

    For Co = 36400 M-1. cm-1

    For Ni = 5520 M-1. cm-1

    molar absorptivity at 656 nm,

    For Co = 1240 M-1. cm-1

    For Ni = 17500 M-1. cm-1

    let [Co] and [Ni] represent the concentration of Co and Ni in diluted solution

    then,

    0.650 = 36400[Co] + 5520[Ni] ... at 510 nm

    0.588 = 1240[Co] + 17500[Ni] ... at 656 nm

    Solving the two equations,

    [Co] = 1.3 x 10^-5 M

    [Ni] = 3.3 x 10^-5 M

    Concentration in 25 ml aliquot,

    [Co] = 1.3 x 10^-5 M x 50 ml/25 ml = 2.6 x 10^-5 M

    [Ni] = 3.3 x 10^-5 x 50 ml/25 ml = 6.6 x 10^-5 M

    concentration (ppm) in original solution prepared by dissolving sample,

    [Co] = 2.6 x 10^-5 M x 250 ml x 58.93 g/mol x 1000/25 ml = 15.322 ppm

    [Ni] = 6.6 x 10^-5 M x 250 ml x 58.69 g/mol x 1000/25 ml = 38.735 ppm
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