Answer:
[tex]\large \boxed{28 \, \% }[/tex]
Explanation:
magnesium + nitrogen ⟶ Product
13 g 5 g
Mass of product = 13 g + 5 g = 18 g
The product contains 5 g of nitrogen .
[tex]\text{Percent N} = \dfrac{\text{5 g}}{\text{18 g}} \times \, 100\% = \mathbf{28 \, \%}\\\text{The percent by mass of nitrogen is $\large \boxed{\mathbf{28 \, \% }}$}[/tex]