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Using rounded atomic masses, what percent by mass of O is in methanol (CH4O)?

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A) 60.0%

B) 40.0%

C) 50.0%

D) 25.0%

Answer: C

Explanation: The formula mass is about 32 g/mol; O contributes 16 g per mole. Divide 16 by 32 and multiply by 100 to obtain about 50.0%.

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