19. In the photocolorimetric determination of Fe by the comparison method, the optical density of a standard solution containing 0.175 mg Fe3+ is 0.248. What is the mass fraction (%) of iron in the ore if its sample equal to 0.2000 g is dissolved in 100.00 ml, 0.5 ml of the solution is taken for analysis and the optical density of the resulting solution after adding all reagents is 0.200?

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