Q.Which of the following statements is correct about the reaction given below: $4Fe(s) + 3O_2(g) \rightarrow 2Fe_2O_3(g)$

The key idea here is the Law of Conservation of Mass.

In any chemical reaction, mass is neither created nor destroyed. This means the total mass of all reactants consumed must be equal to the total mass of all products formed.

1. The Law of Conservation of Mass dictates that the sum of the masses of the reactants ($4Fe(s) + 3O_2(g)$) must equal the sum of the masses of the products ($2Fe_2O_3(g)$).
2. Statement (i) correctly articulates this principle: "Total mass of iron and oxygen in reactants = total mass of iron and oxygen in product therefore it follows law of conservation of mass."
3. Statement (ii) incorrectly links the conservation of mass to the Law of Multiple Proportions, which describes how elements combine in different compounds. …

Understanding the Core Concept: Law of Conservation of Mass

At the heart of any chemical reaction lies the Law of Conservation of Mass. This fundamental law, proposed by Antoine Lavoisier, states that mass is neither created nor destroyed in any chemical reaction. This means that the total mass of all the reactants consumed in a chemical reaction must be exactly equal to the total mass of all the products formed.

A balanced chemical equation is a direct representation of this law. The coefficients in a balanced equation ensure that the number of atoms of each element on the reactant side is equal to the number of atoms of that same element on the product side. Since atoms are conserved, their total mass must also be conserved.

Let's analyze the given reaction and the provided statements based on this and other relevant chemical principles.

The reaction is:

$4Fe(s) + 3O_2(g) \rightarrow 2Fe_2O_3(g)$

To evaluate the statements, we will use the approximate atomic masses commonly used in calculations:

• Atomic mass of Iron (Fe) $\approx 56 \text{ g/mol}$
• Atomic mass of Oxygen (O) $\approx 16 \text{ g/mol}$

Step-by-Step Analysis of the Options

1. Evaluate Option (i): "Total mass of iron and oxygen in reactants = total mass of iron and oxygen in product therefore it follows law of conservation of mass."

   To check this, we calculate the total mass of reactants and products based on the stoichiometry of the balanced equation.

   • Mass of Reactants:

     • Mass of $4Fe$: $4 \times (\text{atomic mass of Fe}) = 4 \times 56 \text{ g/mol} = 224 \text{ g}$
     • Mass of $3O_2$: $3 \times (\text{molecular mass of } O_2) = 3 \times (2 \times 16 \text{ g/mol}) = 3 \times 32 \text{ g/mol} = 96 \text{ g}$
     • Total mass of reactants = $224 \text{ g} + 96 \text{ g} = 320 \text{ g}$

   • Mass of Products:

     • Mass of $2Fe_2O_3$: $2 \times (\text{molecular mass of } Fe_2O_3)$
     • Molecular mass of $Fe_2O_3 = (2 \times \text{atomic mass of Fe}) + (3 \times \text{atomic mass of O})$
     • Molecular mass of $Fe_2O_3 = (2 \times 56) + (3 \times 16) = 112 + 48 = 160 \text{ g/mol}$
     • Total mass of products = $2 \times 160 \text{ g/mol} = 320 \text{ g}$

   Since the total mass of reactants ($320 \text{ g}$) equals the total mass of products ($320 \text{ g}$), the reaction indeed follows the Law of Conservation of Mass. Therefore, statement (i) is correct.

   Law of Conservation of Mass:

   $\text{Total mass of reactants} = \text{Total mass of products}$

2. Evaluate Option (ii): "Total mass of reactants = total mass of product; therefore, law of multiple proportions is followed."

   The first part of the statement, "Total mass of reactants = total mass of product," is correct, as established in step 1. However, the conclusion, "therefore, law of multiple proportions is followed," is incorrect.

   The Law of Multiple Proportions applies when two elements combine to form more than one compound. It states that if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers.

   In the given reaction, iron and oxygen combine to form only one product, $Fe_2O_3$. This reaction itself does not demonstrate the Law of Multiple Proportions. While the Law of Conservation of Mass is always followed in a chemical reaction, it does not imply the Law of Multiple Proportions.

3. Evaluate Option (iii): "Amount of $Fe_2O_3$ can be increased by taking any one of the reactants (iron or oxygen) in excess."

   This statement deals with the concept of limiting and excess reactants. In a chemical reaction, the limiting reactant is the one that is completely consumed first, thereby determining the maximum amount of product that can be formed. The excess reactant is the one present in a greater amount than required to react with the limiting reactant. …

Concept: Laws of Chemical Combination & Limiting Reagent

Method: Law of Conservation of Mass & Stoichiometric Analysis

Step 1: Check the balanced equation

(Note: The product is solid $Fe_2O_3$, not gas — the given equation has a typo)

Step 2: Verify statement (i) — Law of Conservation of Mass

• Law: Mass is neither created nor destroyed in a chemical reaction.
• Check: Count atoms on both sides:
  • Reactants: 4 Fe, 6 O
  • Products: 4 Fe, 6 O
• Conclusion: Mass is conserved. Statement (i) is correct in principle.

Step 3: Verify statement (ii) — Law of Multiple Proportions

• This law applies when two elements form more than one compound (e.g., CO and CO₂).
• Here, only one compound ($Fe_2O_3$) is formed.
• Conclusion: Statement (ii) is incorrect.

Step 4: Verify statements (iii) and (iv) — Limiting Reagent Concept

• The reaction follows a fixed ratio: 4 moles Fe : 3 moles O₂ : 2 moles Fe₂O₃.
• If one reactant is in excess, the other becomes the limiting reagent. …

Here’s a breakdown of the common mistakes students make with this question and how to avoid each.

Mistake 1: Confusing the Law of Conservation of Mass with the Law of Multiple Proportions

What students do wrong:

They see “total mass of reactants = total mass of product” and immediately tick option (ii), thinking it’s the law of multiple proportions.

Why it’s wrong:

The law of multiple proportions deals with the ratios of masses of one element that combine with a fixed mass of another element to form different compounds. It does not say “total mass of reactants = total mass of product.” That statement is actually the law of conservation of mass.

How to avoid:

• Memorise the exact wording of each law:
  • Conservation of mass: Mass is neither created nor destroyed in a chemical reaction → total mass of reactants = total mass of products.
  • Multiple proportions: When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a simple whole-number ratio.
• In this reaction, only one product ($Fe_2O_3$) is formed, so the law of multiple proportions does not apply at all.

Correct approach for option (i):

Check if the equation is balanced.

• Left: 4 Fe, 6 O
• Right: 4 Fe, 6 O Yes, it’s balanced → mass is conserved. So (i) is correct.

Mistake 2: Thinking “excess reactant” always increases product

What students do wrong:

They assume that adding more of any reactant will increase the amount of product. So they pick option (iii).

Why it’s wrong:

In a chemical reaction, the limiting reactant determines how much product is formed. If you already have enough of one reactant, adding more of it will not increase the product — the other reactant becomes the limiting factor.

How to avoid:

• Remember: Only the limiting reactant limits the product.
• Adding excess of the reactant that is already in excess does nothing to the yield.
• In this reaction, if oxygen is in excess, adding more oxygen won’t increase $Fe_2O_3$ — iron is the limiting reactant.

Correct approach for option (iii):

It says “by taking any one of the reactants in excess.” This is false because if you take the wrong one (the one already in excess), product doesn’t increase. So (iii) is incorrect.

Mistake 3: Believing excess reactant decreases product

What students do wrong:

They think adding extra reactant will “use up” the other reactant or somehow reduce the yield, so they pick option (iv).

Why it’s wrong: …