An aqueous solution of a metal nitrate (“X”NO3) forms a precipitate when magnesium sulfate (MgSO4) is added. However, a separate sample of the metal nitrate does not form any precipitate when mixed with hydrogen perchlorate (HClO4). What could this metal “X” be?

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Correct Answer: C. Lead (Pb).
First of all, we can immediately disregard answers A, B, and E. Nearly all compounds containing sodium, potassium, or zinc are soluble. Thus, we are left with answers C and D. Of the two, we realize that sulfur does not bind with sulfate. Sulfur sulfate does not exist. We recognize as well that lead compounds are rarely soluble, but lead perchlorate is. Thus, answer C is the correct answer.
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