IV. Select the correct answer and show steps for following calculations: 1. Fluo

Question

IV. Select the correct answer and show steps for following calculations: 1. Fluorine-18 undergoes positron emission with a half-life of 1.10 x 102 minutes. If a patient is given a 248 mg dose for a PET scan, how long will it take for the amount of fluorine-18 to drop to 83 mg? (Assume that none of the fluorine is excreted from the body.) 2. The nuclide As-76 has a hall-life of 26.0 hours. If a sample of As-76 weighs 344 g, what mass of As-76 remains after 538 minutes? 3. Determine the binding cnergy per nuclcon of an Mg-24 muclcus. The Mg-24 nucleus has a mass of 23.985045. A proton has a mass of 1.00728 amu, a neutron has a mass of 1.008665 amu, and 1 amu is equivalent to 9315 MeV of energy.

Explanation / Answer

Ans 1

Flourine-18 undergoes the first order reaction

Rate constant k = 0.6932/t1/2

= 0.6932/(1.10*10^2 min)

= 0.0063 min-1

ln ( [A] / [A0]) = - k x t

[A] = final concentration after time t = 83 mg

[A0] = initial concentration = 248 mg

ln (83/248) = - 0.0063 x t

-1.095 = - 0.0063 x t

t = 173.74 min

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