135. Solving a chemistry problem

19. Prepared four standard solutions of salts of iron (II), containing 1.00; 1.05; 1.10; 1.15 mg iron. The optical density of the colored solutions with o-phenanthroline, measured relative to the first solution, was equal to 0.24, 0.51 and 0.75, respectively. The optical density of the studied colored solution, measured under the same conditions, was equal to 1.20. Determine the iron content in the test solution (vmg), if 1/5 of the solution was used to prepare the colored solution.