The following information is to be used for the next two questions. Molar absorp

Question

The following information is to be used for the next two questions. Molar absorptivity data for the cobalt and nickel complexes with 2,3-quinoxalinedithiol are Co = 36400 and Ni = 5520 at 510 nm and Co = 1240 and Ni = 17500 at 656 nm. A 0.325-g sample was dissolved and diluted to 150.0 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 0.900 at 510 nm and 0.442 at 656 nm in a 1.00-cm cell.

Calculate the concentration of cobalt (in ppm) in the initial solution prepared by dissolving the sample.

Calculate the concentration of nickel (in ppm) in the initial solution prepared by dissolving the sample.

I have already calculated that the concentration of Ni was 2.376E-5 M and the concentration of Co was 2.11E-5 M, but I need to convert these concentrations into ppm. Help, please.

Explanation / Answer

Dear Friend

1) Molarity (M) = moles / Litre,   moles = weight of solute / Molar Mass of solute

1 ppm = 0.001g solute per 1 kg solution

For water Note that the solution density is assumed to be 1.00 g/mL. This is the common practice in ppm calculations.

Hence for water 1 ppm = 1 g solute per 1,000,000 g solution

2) Convert mol/L to to g/L of solution: weight = molarity X Molar Mass

For Ni : weight = (2.376 X 10-5) X 58.69 = 1.3944 X 10-3 g/L

For Co: weight = (2.11 X 10-5) X 58.93   = 1.2434 X 10-3 g/L

3) Convert to ppm by multiplying numerator and denominator by 1000

For Ni : (1.3944 X 10-3) X 1000/1000 = 1394.4 / 1000000 = 1394.4 ppm

For Co: (1.2434 X 10-3) X 1000/1000 = 1243.4 X 1000000 = 1243.4 ppm

Thank You, all the best.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

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