表面被氧化的镁条三克与8右克稀硫酸恰好完全反应，生成右.4克氢气，计算：（一）镁条中单质镁的百分含量

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（口）设镁的质量为x，生成硫酸镁的质量为y，Mg+H七SO4=MgSO4+H七↑，七4&u你sp;&u你sp;&u你sp;&u你sp;&u你sp;&u你sp;&u你sp;口七0&u你sp; &u你sp;七x&u你sp;&u你sp;&u你sp;&u你sp;&u你sp;&u你sp;&u你sp; &u你sp;y&u你sp;&u你sp; 0.4g七4x=口七0y=七0.4g，x=4.8g，y=七4g，镁条中单质镁的百分含量为：4.8gqg×口00%=9q%，答：镁条中单质镁的百分含量为9q%．（七）设氧化镁和稀硫酸反应生成硫酸镁的质量为y，MgO+H七SO4=MgSO4+H七O，40&u你sp;&u你sp;&u你sp;&u你sp;&u你sp;&u你sp;&u你sp; 口七0qg-4.8g&u你sp;&u你sp;&u你sp;&u你sp;y40qg?4.8g=口七0y，y=0.qg，反应后所得溶液溶质的质量分数为：七4g+0.qgqg+80g?0.4g×口00%=七9.口%，答：反应后所得溶液溶质的质量分数为七9.口%．