设镁带中单质镁的质量为x．
Mg+2HCl=MgCl₂+H₂↑
24            2
x            0.4g

24

x

＝

2

0.4g

∴x=4.8g
∴m_MgO=5g-4.8g=0.2g 
∴m_已变质Mg=0.2g×

24

24+16

=0.12g 
∴m_Mg总=4.8g+0.12g=4.92g 
答：镁带在变质前的质量为4.92 g．