passage 41 hyperlearning mcat science workbook 2006edition

[medscholhopeful]

hi.. the question-#6 reads 1g of insoluble Pb^131I^2 containing only radioactive ^131I was added to 100ml of water. then one gram of soluble NaI(with ^127I) was added to the solution and completely dissolved. Which one of the following statments is correct?

Okay so i narrowed it down to two choice of which i chose A- the addition of Na^127 I willdecrease the amount of the common ion effect but alas that was the wrong choice. The right answer was C which said the molar concentration of ^131Pb I^2 dissolved in solution will increase after the addition of Na^127 I beacuse of the dynamic equilibrium in the solution??? i just don't get it can someone please explain! thanks

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[loveoforganic]

I could see how the concentration of the radioactive isotope of iodine would increase, but not the increase of the lead ii ion. Don't think that's what the answer's saying though.