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 0.227-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 0.542 at 510 nm and 0.396 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 weight percent of cobalt (FM = 58.933 g/mol) and copper (FM = 63.546 g/mol) in the 0.227-g sample?

Explanation / Answer

Molar absorptivity of Co at 510 nm is much greater than Cu. The peak at this wavelength is due to the presence of Co ions.

1. A = cl

or, 0.542 = (3.6390*104 M-1cm-1)(1 cm)*c

c = 1.5 * 10-5 M

For Cu,

0.396 = (1.75*104 M-1cm-1)(1 cm)*c

c = 2.3*10-5 M

2. 0.227 g sample was dissolved to make a solution of 100 mL .

50 mL of this was taken to dilute to 100 mL. So, in the diluted solution amount of sample = 0.227/2 = 0.1135 g

Concentration of Co in solution = 1.5 * 10-5 M

100 mL dilute solution contains

= (1.5 * 10-5 ) * 100/ 1000

= 1.5 * 10-6 M Co

=( 1.5 * 10-6 * 58.933) g Co

= 8.8*10-5 g Co

% of Co in sample =( 8.8*10-5/ 0.1135 ) = 0.08%

Concentration of Cu in solution = 2.3*10-5 M

100 mL dilute solution contains

= (2.3*10-5) * 100/ 1000

= 2.3*10-6 M Co

=( 2.3*10-6 * 63.546) g Co

= 1.46*10-4 g Co

% of Co in sample =( 1.46*10-4 / 0.1135 ) = 0.13 %

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