A rock sample containing Cu was chemically digested; the resulting solution was

Question

A rock sample containing Cu was chemically digested; the resulting solution was made up to 100 mL in a volumetric flask. 10 mL of this solution was treated with appropriate reagents to result in 20mL of colored solution. The absorbance of this solution was measured at 454 nm in a 1 cm cell. The resulting absorbance was 0.393. A blank carried through the same procedure gave an absorbance of 0.045. If molar absorptivity of the Cu-containing colored compound is 7900, what is the amount of Cu in the rock sample (in mg)?

Explanation / Answer

Beer's law: A = ecl

whete A is absorbance, e is molar absorptivity, c is concentration and l is path length

A = A(sample) - A(reference) = 0.393 - 0.045 = 0.348

Concentration = A/el

= 0.348/(7900 x 1) = 4.405 x 10^(-5) M

Moles of Cu in 10 mL of solution = moles of Cu in 20 mL of colored solution

= volume x concentration of colored solution

= 20/1000 x 4.405 x 10^(-5) = 8.810 x 10^(-7) mol

Moles of Cu in sample = moles of Cu in 100 mL of solution

= 100/10 x 8.810 x 10^(-7) = 8.810 x 10^(-6) mol

Mass of Cu in sample = moles x molar mass of Cu

= 8.810 x 10^(-6) x 63.546

= 5.60 x 10^(-4) g = 0.560 mg

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