9. An unknown amount of lead(II) solution with a volume of 15.00 ml is titrated with a 0.0010 M EDTA solution in the presence of 4-(2-pyridylazo)-resorcinol at pH 9.5 and λ=516 nm to a constant value A=0.15 . The optical densities corresponding to the added titrant volumes of 2.5 and 3.5 ml are 0.65 and 0.25, respectively. Calculate the mass of lead in solution. (0.774 mg)

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