145. Solving a chemistry problem

145. Solving a chemistry problem

35. When photocolorimetric determination of Fe by comparison method, the optical density of a standard solution containing 0.75 mg of Fe is 0.248. What is the mass fraction (%) of iron in the ore, if its weight equal to 0.2000 g is dissolved in 100.00 ml, 0.5 ml of the solution and the optical density of the obtained solution after the addition of all reagents and dilution to 100 cm3 (as and for a standard solution) equal to 0.200?