Ligand X forms a complex with both cobalt and copper, each of which has a maximu
ID: 589721 • Letter: L
Question
Ligand X forms a complex with both cobalt and copper, each of which has a maximum absorbance at 510 nm and 645 nm, respectively. A 0.280-g sample containing volume of 100.0 mL: A solution containing ligand X was added to a 50.0 mL aliquot of the sample solution and diluted to a final volume of 100.0 mL. The measured absorbance of the unknown solution waS 0.416 at 510 nm and 0.304 at 645 nm, when measured with a 1.00-am cell. The molar absorptivities of the cobalt and copper complexes at each wavelength are shown in the table below cobalt and copper was dissolved and diluted to a wavelength A, nm 510 645 Molar Absorptivity (e, M-1cm-1) Co 3.7040 x101 1262 Cu 5617 17810 What is the concentration of cobalt and copper in the final diluted solution? Number Number What is the weight percent of cobalt (FM -58.933 gímol) and copper (FM -63.546 g/mol) in the 0.280-g Number Number I wt % Co2+-- % wt% Cu"-Explanation / Answer
Absorbance of solution = absorbances of all components in solution
Absorbance = molar absorptivity x path length x concentration
with path length = 1 cm
we have,
at 510 nm,
0.416 = 3.704 x 10^4[Co] + 5617[Cu]
at 645 nm,
0.304 = 1262[Co] + 17810[Cu]
Solving,
Concentration of cobalt and cupper in final diluted solution,
[Co2+] = 8.74 x 10^-6 M
[Cu2+] = 1.64 x 10^-5 M
moles of Co2+ in original solution = 8.74 x 10^-6 M x 100 ml/50 ml = 1.75 x 10^-5 M x 0.1 L = 1.75 x 10^-6 mol
mass of Co2+ in original solution = 1.75 x 10^-6 mol x 58.933 g/mol = 0.000103 g
wt% Co2+ = 0.000103 g x 100/0.280 g = 0.037%
moles of Cu2+ in original solution = 1.64 x 10^-5 M x 100 ml/50 ml = 3.28 x 10^-5 M x 0.1 L = 3.28 x 10^-6 mol
mass of Cu2+ in original solution = 3.28 x 10^-6 mol x 63.546 g/mol = 0.00021 g
wt% Cu2+ = 0.00021 g x 100/0.280 g = 0.074%
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