Reacting masses: cross the mole bridge

Plot twist: a balanced equation is a recipe written in moles.

$2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}$

This says 2 mol of magnesium reacts with 1 mol of oxygen to make 2 mol of magnesium oxide. It does not say 2 g + 1 g → 2 g. (That would be 3 g in and 2 g out, and conservation of mass would like a word.)

The coefficients give the mole ratio of the reaction, its stoichiometry. Here Mg : $\text{O}_2$ : MgO = 2 : 1 : 2.

1. Mass → moles of the substance you know: $n = \dfrac{m}{M}$.
2. Mole ratio: use the coefficients to get moles of the substance you want.
3. Moles → mass: $m = n \times M$.

Insider hack for step 2: write the ratio as wanted : known, then multiply by wanted ÷ known. For $2\text{Mg} \rightarrow 2\text{MgO}$ it's 2 : 2, so ×1. For $\text{Fe}_2\text{O}_3 \rightarrow 2\text{Fe}$ it's 2 : 1, so ×2.