79. Solving a chemistry problem

2. A portion of pyrite 0.556 g was dissolved, and the resulting solution was diluted with water to 250 ml; from 5 ml of this solution, after appropriate treatment, 50 ml of a suspension of BaSO4 was obtained, the optical density of which was found to be 0.45. To build a calibration curve, the following data were obtained (concentration - calculated as SO3)
SO3 concentration, mg/ml 0.05 0.10 0.16 0.22 0.27
Optical density 0.1 0.2 0.3 0.4 0.5
Determine the sulfur content in pyrite (mass fraction, %).