4. In volumetric flasks with a volume of 25.00 ml, 6.00 and 9.00 ml of a standard solution of iron(III) salt with a titer of 0.01000 mg/ml were placed. 20.00 ml of analyzed waste water was introduced into the third flask. After adding the photometric reagent and diluting the solutions with water to the mark, the optical densities of the solutions were 0.33, respectively; 0.68; 0.51. Calculate the mass concentration of iron (mg/l) in waste water.

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