Think of it like a kitchen recipe: if you mix $50 \text{ g}$ of flour with $20 \text{ g}$ of sugar, you will always get $70 \text{ g}$ of mixture; nothing disappears into thin air.

L-A-M: Lavoisier, Always Matches (Mass in = Mass out).

The Law of Conservation of Mass, proposed by Antoine Lavoisier, states that matter is neither created nor destroyed during a chemical reaction. In this reaction, $2Na + Cl_{2} \rightarrow 2NaCl$. Since the total mass of reactants is $5.6 \text{ g} + 8.0 \text{ g} = 13.6 \text{ g}$, the resulting product must also weigh $13.6 \text{ g}$.

Always sum the reactants given in the problem statement first; if they equal the mass of products, the law is satisfied regardless of complex stoichiometry.