29. Solving a chemistry problem

1. When determining chromium by the calibration curve method, 10.0 was placed in volumetric flasks with a capacity of 250 ml; 11.0; 12.0; 13.0; 14.0 ml of standard solution. containing 1.25 mg/ml Cr, oxidized chromium to dichromate and diluted with water to the mark. The optical densities of solutions measured in 20 mm cuvettes at λ=432 nm are:
V, ml 10.0; 11.0; 12.0; 13.0; 14.0
А432 0.12 0.20 0.32 0.40 0.43
A sample of ore weighing 0.5500 g was dissolved, chromium was oxidized to dichromate, diluted to 1000 ml. 50 ml of the solution was taken, diluted to 250 ml, and the optical density was measured under the same conditions. Calculate the mass fraction of chromium (%) in the ore if the optical density is 0.27.