7. To determine antimony traces in lead, the method of double isotope dilution was used. A sample of lead with a mass of m u was dissolved in a mixture of nitric and tartaric acids. The resulting solution was divided into two equal parts, and various additives of radioactive antimony (m1 and m2) were introduced into each of them. Lead was precipitated as PbCl2, and antimony was oxidized to Sb(V). A substoichiometric amount of methyl violet was added to each portion of the solution, the antimony bound in the complex was extracted with toluene, and the activity of each portion (A1 and A2) was measured. Calculate the mass fraction (%) of antimony in lead if
Option 1 2 3
m, g 1 5 10
m1, μg 12 6 6
m2, μg 24 12 12
A1, cpm 282 35 380
A2, cpm 386 42 450
Solution for option 1

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