I added 4 mL of a 20 ppm copper solution to 10 mL of an solution that had an unk

Question

I added 4 mL of a 20 ppm copper solution to 10 mL of an solution that had an unknown amount of copper concentration (and that has an absorbance of 0.1300), then diluted to 50 mL. The absorbance of mixture was 0.0472. What is the concentration in ppm of the mixture? I also made 4 other solutions: 1.) 10 mL of the unknown with 0 mL of the 20 ppm solution and an absorbance of 0.0251
2.) 10 mL of the unknown with 8 mL of the 20 ppm solution and an absorbance of 0.0713
3.) 10 mL of the unknown with 12 mL of the 20 ppm soltuion and an absorbance of 0.0934
4.) 10 mL of the unknown with 16 mL of the 20 ppm solution and an absorbance of 0.1128.
Each of these solutions were diluted to 50 mL after being mixed

Explanation / Answer

Concentration of Cu in standard = 20 ppm

concentration of 4 ml Cu std in diluted solution = 4 x 20/50 = 1.6 ppm

Absorbance due to Cu standard in a sample = 1.6 ppm x 0.1300/20 ppm = 0.0104

Actual absorbance of sample = 0.0472

Absorbance due to unknown = 0.0472 - 0.0104 = 0.0368

concentraion of unknown in diluted solution = 0.0368 x 20ppm/0.1300 = 5.66 ppm

Concentration of Cu in the unknown (10 ml sample) = 5.66 ppm x 50 ml/10 ml = 28.3 ppm

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