To spectrophotometrically determine the mass percent of cobalt in an ore containing cobalt and some — Spectrophotometry Chemistry Question
Problem Context
To spectrophotometrically determine the mass percent of cobalt in an ore containing cobalt and some inert materials, solutions with known [Co2+] are prepared and the absorbance of each of the solutions is measured at the wavelength of optimum absorbance. The data are used to create a calibration plot, shown below.
[VISUAL]
A 0.630 g sample of the ore is completely dissolved in concentrated HNO3(aq). The mixture is diluted with water to a final volume of 50.00 mL. Assume that all the cobalt in the ore sample is converted to Co2+(aq).
Model Answer
An absorbance of 0.74 corresponds to a concentration of 0.0130 M (acceptable range: 0.0125 M to 0.0135 M).
1 point is earned for a concentration that is consistent with an absorbance of 0.74 on the graph.
Model Answer
0.05000 L * (0.0130 mol Co2+ / 1.00 L) = 6.50 * 10^-4 mol
1 point is earned for the correct calculation of the number of moles.
Model Answer
6.50 * 10^-4 mol * (58.93 g Co / 1.00 mol) = 0.0383 g Co
(0.0383 g Co / 0.630 g ore) * 100% = 6.08% Co
1 point is earned for a correct calculation of the number of grams of Co that is consistent with the number of moles of Co2+ in part (b) (may be implicit).
1 point is earned for a correct calculation of the mass percent that is consistent with the number of grams of Co.