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Using rounded atomic masses, what percent by mass of C is in ethane (C2H6)?

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A) 20.0%

B) 80.0%

C) 90.0%

D) 70.0%

Answer: B

Explanation: The formula mass is about 30 g/mol; C contributes 24 g per mole. Divide 24 by 30 and multiply by 100 to obtain about 80.0%.

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