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锌粉、铝粉、铁粉、镁粉的混合物a克与一定质量的质量分数为25%的稀硫酸恰好完全反应，蒸发水分后得到固体c（不含结晶水）b克。求反应过程中生成氢气多少克？

25%的H2SO4恰好完全反应，而氢元素全部由H2SO4提供，所以以H2SO4的量来计算H2的量
而蒸发水分后得固体（不含结晶水）b g——它是由锌、铝、铁、镁这几种金属元素与硫酸根离子组合而成，b-a即为所得固体中硫酸根离子的质量
SO4*2－ ～ H2SO4 ～ H2
96                                     2
b-a                                   m
96/(b-a)=2/m