Answer
In a balanced chemical equation, the number "3" before \( \mathrm{FeCl}_2 \) means there are **3 moles** of iron(II) chloride involved in the reaction. This ensures the equation is balanced and follows the law of conservation of mass.
Solution
In a balanced chemical equation, the term "\( 3 \mathrm{FeCl}_2 \)" serves a specific purpose related to the quantities of substances involved in the reaction. Here's what it signifies:
1. **Stoichiometric Coefficient**: The number **3** placed before \( \mathrm{FeCl}_2 \) is known as a *stoichiometric coefficient*. This coefficient indicates the number of **moles** (or proportional units) of iron(II) chloride (\( \mathrm{FeCl}_2 \)) that participate in the reaction.
2. **Balancing the Equation**: Chemical equations must be balanced to satisfy the **Law of Conservation of Mass**, which states that matter cannot be created or destroyed in a chemical reaction. The coefficient ensures that there are equal numbers of each type of atom on both the reactant and product sides of the equation.
3. **Quantitative Relationships**: The coefficient establishes the **ratio** in which reactants combine and products form. For example, if the balanced equation is:
\[
3 \mathrm{FeCl}_2 + 2 \mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{Fe}_2(\mathrm{SO}_4)_3 + 6 \mathrm{HCl}
\]
This tells us that **3 moles of iron(II) chloride** react with **2 moles of sulfuric acid** to produce **1 mole of iron(III) sulfate** and **6 moles of hydrochloric acid**.
4. **Molecular Quantity**: In contexts where the equation represents molecules (e.g., in gas-phase reactions), the coefficient indicates the number of **molecules**. So, "3\( \mathrm{FeCl}_2 \)" would mean three molecules of iron(II) chloride.
**Summary**: The coefficient "3" before \( \mathrm{FeCl}_2 \) indicates that three units (moles or molecules, depending on the context) of iron(II) chloride are required to balance the chemical reaction, ensuring that the law of conservation of mass is upheld.
**Example**:
If the balanced equation is:
\[
3 \mathrm{FeCl}_2 + 2 \mathrm{H}_3\mathrm{PO}_4 \rightarrow \mathrm{Fe}_3(\mathrm{PO}_4)_2 + 6 \mathrm{HCl}
\]
It means:
- **3 moles** of \( \mathrm{FeCl}_2 \) react with **2 moles** of \( \mathrm{H}_3\mathrm{PO}_4 \)
- Producing **1 mole** of \( \mathrm{Fe}_3(\mathrm{PO}_4)_2 \) and **6 moles** of \( \mathrm{HCl} \)
Understanding these coefficients is essential for performing accurate stoichiometric calculations in chemistry.
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