135. Solving a chemistry problem

11. The deviation of the galvanometer of a single-beam photocolorimeter in the study of a solution of K2Cr2O7 is 75 divisions of the scale. After adding 1 ml of standard 0.015 N. potassium bichromate solution at a constant layer thickness, the deviation was 65 divisions of the scale. Determine the concentration of the test solution (in mg / ml), if the deviation on the galvanometer scale for the background is 100 divisions.