NaOH少量Mg(HSO4)2 + 2 NaOH = MgSO4 + Na2SO4 + 2 H2O
离子方程式H+ + OH- = H2O

NaOH过量Mg(HSO4)2 + 4 NaOH = Mg(OH)2 ↓ + 2 Na2SO4 + 2 H2O
离子方程式Mg2+ + 2 H+ + 4 OH- = Mg(OH)2 ↓ + 2 H2O

要先中和H+,否则即使生成了Mg(OH)2 也被H+溶解了