126. Solving a chemistry problem

2. When analyzing a sample weighing 0.9816 g for the content of cobalt by the chemiluminescent photographic method on 1 plate, the intensity of the luminescence of a sample of the analyzed solution, standards and a blank experiment was measured.
0.5 ml of a cobalt salt solution was placed into the cells of the cuvette, sodium salicylate was added (to eliminate the interfering effect of Cu and Fe cations) and the same amount of H2O2. Then the cuvette was kept until the complete cessation of the glow; the plate was photometered on an MF-2 microphotometer.
The value of illumination ∆S of standard solutions containing 4.00 8.00 12.00 16.00 µg/ml was 0.17 0.28 0.40 0.23, respectively.
Calculate the mass fraction (%) of Co in the sample if ∆Sx=0.20.