135. Solving a chemistry problem

15. To determine iron in wastewater, a sample of 0.0586 g of pure iron oxide containing 2.5% hygroscopic water was dissolved in acid and the volume of the solution was adjusted to 250 ml. To construct a calibration graph, an ammonium thiocyanate solution was added to the volumes of the resulting solution indicated below, the solutions were diluted with water to 100 ml, and colorimetrically measured on a photocolorimeter with a diaphragm. The following aperture scale readings were obtained:
Solution volume, ml 0.5 1.0 1.5 2.0 2.5 3.0
Photocolorimeter readings 17 31 42 51 58 64
2.5 l of waste water was evaporated to 50 ml and 15 ml of this solution, after adding the ammonium thiocyanate solution, was brought to 25 ml with water. For the resulting colored solution, the reading on the aperture scale was 25 divisions.
Determine the iron content in wastewater (in g/m3).