$\Delta m_{dd}=6,6g=m_R-m_{H_2}$
Bảo toàn R: $n_R=n_{RCl_2}=\dfrac{28,5}{R+71}$
Bảo toàn e: $n_{H_2}=n_R$
$\Rightarrow \dfrac{28,5R}{R+71}-\dfrac{2.28,5}{R+71}=6,6$
$\Leftrightarrow \dfrac{28,5R-57}{R+71}=6,6$
$\Leftrightarrow 28,5R-57=6,6(R+71)$
$\Leftrightarrow R=24(Mg)$
$\to R$ là magie