78. Solving a chemistry problem

5. To determine nickel with dimethylglyoxime, a sample of steel is dissolved and the solution is diluted to 200.0 ml. The necessary reagents are added to 5.00 cm3 of the solution, the solution is diluted with water to 50.0 cm3 and photometrically measured at an effective wavelength of 470 nm in a cuvette with a light-absorbing layer thickness of 2.0 mm. The molar coefficient of light absorption of nickel dimethylglyoxime at this wavelength is 13000. Calculate the mass of the steel sample for analysis if the optimal value of optical density is 0.435 and the approximate mass fraction (%) of nickel in steel is 0.5%.