Radioactive tracers are useful in studying very low concentrations of chemical species.A chemist has a sample of HgI₂ in which part of the iodine is the radioactive nuclide of mass 131,so that the count rate is 5.0 $\times$10¹¹ counts per minute per mole of I.The solid mercuric iodide is placed in water and allowed to come to equilibrium.Then 100 mL of the solution is withdrawn,and its radioactivity is measured and found to give 22 counts per minute.What is the molar concentration of iodide ion in the solution? A)1.1 $\times$ 10^-⁹ B)4.4 $\times$ 10^-¹⁰ C)1.1 $\times$ 10^-¹⁰ D)1.1 $\times$ 10^-¹¹ E)4.4 $\times$ 10^-¹¹

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Quizplus · Accepted Answer

The original count rate is 5.0 x 10^11 counts per minute per mole of I. The measured activity in the solution is 22 counts per minute, which is 22 / (5.0 x 10^11) = 4.4 x 10^-11 of the original activity per 100 mL. Since this represents the fraction of a mole of I in 100 mL, to find the molarity (moles per liter), we multiply by 10 (to convert 100 mL to a liter), resulting in 4.4 x 10^-10 M.

Radioactive Tracers Are Useful in Studying Very Low Concentrations of Chemical