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MCAT Question A Day - 3/9/14 - Answer!

A student finds that MgCO3 is more soluble in HNO3 than in plain water. Which of the following is a likely explanation for this effect?

A.  The NO3 - ion attaches preferentially to the Mg2+.
B.  Mg2+ is a strong base and reacts completely with the H+ from the nitric acid, causing the magnesium salt to dissociate.
C.  Carbonate is a weak base and reacts with the H+ from the strong acid, causing the barium salt to dissociate.
D.  The HNO3 reduces the Mg2+ to magnesium metal.

The correct answer is (C). CO32- from the BaCO3 reacts with H+ to form HCO3- (and, if enough strong acid is available, H2CO3), causing the MgCO3 to dissociate more than it would in neutral water.

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