3. To determine copper in the preparation, from a sample weighing 0.325 g after dissolution and treatment with excess ammonia, 250 ml of a colored solution was obtained, the optical density of which in a cuvette with a layer thickness of 20 mm at λ = 620 nm is 0.254. Calculate the mass fraction of copper in the preparation if the molar absorption coefficient of copper ammonia is 120 mol-1*l*cm-1

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