Ligand X forms a complex with both cobalt and copper, each of which has a maximu

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 0.219-g sample containing cobalt and copper was dissolved and diluted to a 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 0.421 at 510 nm and 0.308 at 645 nm, when measured with a 1.00-cm cell. The molar absorptivities of the cobalt and copper complexes at each wavelength are shown in the table below. What is the concentration of cobalt and copper in the final diluted solution? [Co^2+] = [Cu^2+] = What is the weight percent of cobalt (FM = 58.933 g/mol) and copper (FM = 63.546 g/mol) in the 0.219-g sample? wt % Co^2+ wt% Cu^2+

Explanation / Answer

The general rule for calculating concentration of a mixture using beer's law: total absorption is equal to sum of the individual absorption.

At 510nm :

A = 36865[Co] + 5591[Cu]

or, 0.421 = 36865[Co] + 5591[Cu]

or,[Cu] = 0.421-36865[Co]/5591

At 645 nm:

A = 1256[Co] + 17730[Cu]

or, 0.306 = 1256[Co] + 17730[Cu]

substitute the value of [Cu] in the above equation.

0.306 = 1256[Co] + 17730[Cu]

or, 0.306 = 1256[Co] + 17730[0.421-36865[Co]/5591]

or, [Co]= 9*10^-6 M

[Cu ]=1.7*10^-5 M

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