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Using rounded atomic masses, what percent by mass of Na is in sodium hydroxide (NaOH)?

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A) 42.5%

B) 57.5%

C) 67.5%

D) 47.5%

Answer: B

Explanation: The formula mass is about 40 g/mol; Na contributes 23 g per mole. Divide 23 by 40 and multiply by 100 to obtain about 57.5%.

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