\textbf{216.} If from the complete combustion of a certain amount of magnesium under standard conditions, 20 grams of magnesium oxide is formed and 300 kilojoules of heat is released, what is the standard $\Delta H$ of formation of magnesium oxide per mole? $(O = 16,\ Mg = 24 : \text{g.mol}^{-1})$
\medskip
(1) $+300$ \hfill (2) $-300$ \hfill (3) $+600$ \hfill (4) $-600$
\medskip