The name and formula of the compound of magnesium, chlorine and oxygen used as a drying agent is

Magnesium perchlorate, also known as anhydrone, is used as a drying agent as it has strong affinity for water giving $\mathrm{Mg}{\left({\mathrm{ClO}}_4\right)}_2\cdot 6{\mathrm{H}}_2\mathrm{O}$. It loses all water molecules of crystallization, when heated at 250°C.

$\mathrm{Mg}{\left({\mathrm{ClO}}_4\right)}_2\cdot 6{\mathrm{H}}_2\mathrm{O}\stackrel{{250}^{\circ }\mathrm{C}}{\to }\mathrm{Mg}{\left({\mathrm{ClO}}_4\right)}_2+6{\mathrm{H}}_2\mathrm{O}$