TN1 n$Mg$ =0,1 mol
$Mg$ +2$HCl$ => $MgCl_2$ + $H_2$
0,1 mol =>0,1 mol
m cốc=mdd sau pứ=mdd $HCl$ +m$Mg$ -m$H_2$
=m+2,4-0,2=m+2,2g
TN2: 2 $Al$ + 3$H_2SO_4$ => $Al_2(SO_4)_3$ +3$H_2$
a/27 mol =>1,5a/27 mol
m cốc ko đổi=m$Al$+mdd$H_2SO_4$-m$H_2$
=>a+m-3a/27=m+2,2
=>8a/9=2,2=>a=2,475g