3. To build a calibration graph, 250 ml were placed in volumetric flasks, 11.0 was placed; 12.0; 13.0; 14.0;15.0 standard solution containing 1.25 mg/ml Mn and oxidized manganese to permanganate. The optical density was measured relative to a solution containing 12.5 mg Mn in 250 ml and received:
V, ml 11.0 12.0 13.0 14.0 15.0
D 0.200 0.400 0.600 0.800 1.01
A sample of ore weighing 0.500 g was dissolved and the solution was diluted to 1000 ml. In 50 ml of the filtrate, Mn was oxidized to permanganate and the solution was diluted to 250 ml. The static density of the resulting solution is 0.320. Calculate the mass fraction (%) of Mn in the ore.

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