在化学方程式 2Mg + O₂ → 2MgO 中，若镁完全反应，消耗了0.4 mol镁，则消耗氧气的物质的量为多少？
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Correct answer
A. 0.2 mol
Principle or equation
根据化学方程式中的化学计量数之比，反应物之间的物质的量之比等于化学计量数之比。
Why this answer is correct
由方程式 2Mg + O₂ → 2MgO 可知，镁与氧气的物质的量之比为2:1。因此，若消耗0.4 mol镁，则消耗氧气的物质的量为0.4 mol ÷ 2 = 0.2 mol。
Example
若消耗1 mol镁，则消耗氧气0.5 mol；若消耗0.4 mol镁，则消耗氧气0.2 mol。
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