HUKUM KEKEKALAN MASSA ( HUKUM LAVOISIER ) : HUKUM DASAR KIMIA

Foundational Chemical Laws
📌 The development of chemistry is based on several foundational laws, including the Law of Conservation of Mass (Lavoisier), the Law of Definite Proportions (Proust), the Law of Multiple Proportions (Dalton), Gay-Lussac's Law, and Avogadro's Law.
🔬 Lavoisier concluded that mass is conserved in a closed system; the total mass of substances before and after a reaction is equal.
🔥 Phenomena like paper combustion (where ash mass is less than paper mass) occur in open systems because gaseous products escape, leading to an apparent mass loss.

Applying the Law of Conservation of Mass
🔩 In the rusting of iron ($\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$), the mass of the resulting iron oxide is greater than the initial mass of the iron because it incorporates oxygen ($\text{O}_2$) from the air.
🧪 An experiment demonstrating the law involves mixing lead(II) nitrate ($\text{Pb}(\text{NO}_3)_2$) and potassium iodide ($\text{KI}$) in a sealed tube; the total mass before mixing equals the total mass after the reaction ($\text{PbI}_2$ and $\text{KNO}_3$ formation).
⚖️ For the reaction $\text{Pb}(\text{NO}_3)_2 + 2\text{KI} \rightarrow \text{PbI}_2 + 2\text{KNO}_3$, the $\text{mass of } \text{Pb}(\text{NO}_3)_2 + \text{mass of } \text{KI} = \text{mass of } \text{PbI}_2 + \text{mass of } \text{KNO}_3$.

Problem Solving Based on Mass Conservation
🔢 A reaction involving 5g of sulfur and 10g of copper must result in 15g of copper(II) sulfide ($\text{CuS}$), upholding the law.
⚙️ If 32g of copper ($\text{Cu}$) reacts with oxygen ($\text{O}_2$) to form 40g of copper oxide ($\text{CuO}$), the mass of $\text{O}_2$ consumed must be $40\text{g} - 32\text{g} = \mathbf{8\text{g}}$.
🌟 If 4g of magnesium ($\text{Mg}$) reacts with 5g of oxygen ($\text{O}_2$), the resulting magnesium oxide ($\text{MgO}$) will have a mass of $4\text{g} + 5\text{g} = \mathbf{9\text{g}}$.

Key Points & Insights
➡️ Identify the system: Mass conservation only holds true in a closed system; open systems show apparent mass changes due to escaping or entering gases.
➡️ Verify adherence: If reactants mass ($15\text{g}$) results in a product mass ($10\text{g}$), the reaction does not follow the Law of Conservation of Mass under those conditions.
➡️ Calculate unknowns: Use the principle that $\text{Mass}_\text{Reactants} = \text{Mass}_\text{Products}$ to solve for missing reactant or product masses in balanced chemical scenarios.

📸 Video summarized with SummaryTube.com on Jan 25, 2026, 13:41 UTC