49. Solving a chemistry problem

3. Calculate the concentration of cyanocobalamin (in percent) in terms of dry matter.
Method of determination. About 0.1 g (exact weight) of the drug is dissolved in water in a volumetric flask with a capacity of 500 ml and adjusted to the mark with water. Transfer 25 ml of this solution into a 250 ml volumetric flask and dilute to the mark with water. Determine the optical density of the resulting solution on a spectrophotometer at a wavelength of 361 nm in a cuvette with a layer thickness of 10 mm. The reference solution is water.
The content of cyanocobalamin is calculated on a dry basis. The moisture content in it is 12.0%, the optical density is 0.341, E1% 1cm is 207, the exact weight of the preparation is 0.1000 g.