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Using rounded atomic masses, what percent by mass of C is in methane (CH4)?

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A) 25.0%

B) 85.0%

C) 75.0%

D) 65.0%

Answer: C

Explanation: The formula mass is about 16 g/mol; C contributes 12 g per mole. Divide 12 by 16 and multiply by 100 to obtain about 75.0%.

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