Molar absorptivity data for the cobalt and nickel complexes with 2, 3-quinoxalin

Question

Molar absorptivity data for the cobalt and nickel complexes with 2, 3-quinoxalinedithiol are epsilon_Co = 36400 and epsilon_Ni = 5520 at 510 nm and epsilon_Co = 1240 and epsilon_Ni = 7500 at 656 nm. A 0.373-g sample was dissolved and diluted to 50.0 mL. A 25.0-mL aliquot was treated to eliminate interferences: after addition of 2, 3-quinoxalinedithiol, the volume was adjusted to 50.0 mL. This solution had an absorbance of 0.412 at 510 nm and 0.302 at 656 nm in a 1.00-cm cell. Calculate the concentration in parts per million of cobalt and nickel in the sample. Concentration of cobalt = ppm Concentration of nickel = ppm.

Explanation / Answer

A510 = 510(Co) x c(Co) x l + 510(Ni) x c(Ni) x l

A656 = 656(Co) x c(Co) x l + 656(Ni) x c(Ni) x l

0.412 = 36400c(Co) + 5520c(Ni)

0.302 = 1240c(Co) + 17500c(Ni)

c(Co) = 8.79 x 10-6 M, c(Ni) = 1.66 x 10-5 M

Note 25mL taken from 50mL original solution and then diluted to 50mL

Moles of Co in sample = 2 x 50/1000 x 8.79 x 10-6 = 8.79 x 10-7 mol

Moles of Ni in sample = 2 x 50/1000 x 1.66 x 10-5 = 1.66 x 10-6 mol

Mass of Co in sample = 8.79 x 10-7 x 58.93 = 5.18 x 10-5 g = 0.0518mg

Mass of Ni in sample = 1.66 x 10-6 x 58.69 = 9.74 x 10-5 g = 0.0974 mg

Mass of sample = 0.373 g = 0.000373 kg

Concentration of Co = 0.0518/0.000373 = 138.87 ppm

Concentration of Ni = 0.0974/0.000373 = 261.13 ppm

Related Questions

Navigate

• Browse (All)
• Browse M
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.