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Using rounded atomic masses, what percent by mass of Mg is in magnesium carbonate (MgCO3)?

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A) 71.4%

B) 38.6%

C) 18.6%

D) 28.6%

Answer: D

Explanation: The formula mass is about 84 g/mol; Mg contributes 24 g per mole. Divide 24 by 84 and multiply by 100 to obtain about 28.6%.

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