Magnesium burns in nitrogen: $3\,\text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2$. A mixture of $6.0\ \text{g}$ of Mg and $4.0\ \text{g}$ of $\text{N}_2$ is heated, and the reaction goes only to $80\%$ of the extent that the limiting reagent allows. What mass of $\text{N}_2$ is left when the reaction stops? Atomic masses in $\text{g mol}^{-1}$: N 14.0, Mg 24.0.