98. Solving a chemistry problem

7. To determine the thorium content in a solution using the fixed concentration method, we used the oxidation reaction of iodide ion with hydrogen peroxide, catalyzed by thorium compounds. 2.0 ml of the test solution was diluted to 100.0 ml in a volumetric flask, and the resulting solution was diluted another 10 times. Determine the concentration of thorium (g/l) in the test solution, if, when measuring time t, it is necessary to achieve an optical density corresponding to the optical density of a solution with a thorium concentration of 8.2 * 10-6 g/l, the following results were obtained:
Parameters Standard solutions Test solutions
1 2 3 4 5 6 1 2 3
CTh*106, g/l 3.0 4.5 6.0 8.2 9.0 12.0 ? ? ?
t, min 10.0 6.7 5.0 3.7 3.3 2.5 4.3 8.3 2.9