Coi X gồm Ca (x mol), Mg (y mol), O (z mol)
$\Rightarrow 40x+24y+16z=10,72$ (1)
$n_{H_2}=0,145 mol$
Bảo toàn e: $2x+2y-2z=0,145.2=0,29$ (2)
$n_{MgCl_2}= y$
$\Rightarrow 95y=12,35$ (3)
(1)(2)(3)$\Rightarrow x=0,14; y=0,13; z=0,125$
$a= m_{CaCl_2}=0,14.111=15,54g$