# 100 grams of magnesium and 2.50 grams of oxygen react together in synthesis reaction to form magnesium oxide. How do you write this balanced reaction?

Apr 30, 2016

The balanced equation is clearly:

$M g \left(s\right) + \frac{1}{2} {O}_{2} \left(g\right) \rightarrow M g O \left(s\right)$

#### Explanation:

It is abundantly clear that magnesium is in vast molar excess:

i.e. $\text{Moles of metal}$ $=$ $\frac{100 \cdot g}{24.31 \cdot g \cdot m o {l}^{-} 1}$ $\cong$ $5 \cdot m o l$

$\text{Moles of dioxygen}$ $=$ $\frac{2.5 \cdot g}{32.00 \cdot g \cdot m o {l}^{-} 1}$ $\cong$ $0.08 \cdot m o l$

Clearly, ${O}_{2}$ is the limiting reagent, and our calculation proceeds on the basis of the molar quantity of the gas.