4.1.4 Mole-Mole Calculation

This topic explains how to perform mole-mole calculations using a balanced chemical equation. It shows how the coefficients in a balanced equation give the mole ratio between reactants and products.

Definition

Mole-Mole Calculation: A stoichiometric calculation in which the number of moles of one substance is determined from the number of moles of another substance using the mole ratio from a balanced chemical equation.

Main Idea

In a balanced chemical equation, the coefficients are taken as moles. They indicate the proportion of each chemical in the equation. Using this mole ratio, we can calculate how many moles of one substance are needed to react with, or are produced from, a given number of moles of another substance.

Core Concept

For a general balanced equation:

aA + bB → cC + dD

The coefficients a, b, c, and d represent the number of moles of A, B, C, and D respectively.

The mole ratio allows conversion:

Moles of unknown = Moles of known × Coefficient of unknownCoefficient of known

Detailed Explanation

What Does a Balanced Equation Tell Us?

In a balanced chemical equation, the coefficients used are taken as moles that indicate the proportion to the next chemical in the equation.

Example:

CH₄ + 2O₂ → CO₂ + 2H₂O

• For complete burning of one mole of CH₄, 2 moles of O₂ are required.
• 1 mole of methane can produce 2 moles of water and 1 mole of CO₂.

Mole Ratio

From the equation:

Example 4.5: Magnesium Reacting with Oxygen

Problem: How much MgO is formed when 4 moles of magnesium react with excess of O₂? Also calculate moles of O₂ required.

Chemical Equation:

2Mg + O₂ → 2MgO

Solution:

Part (a): Moles of MgO formed

From the equation: 2 moles of Mg produce MgO = 2 moles

1 mole of Mg produces MgO = 22 = 1 mole

4 moles of Mg produce MgO = 22 × 4 = 4 moles of MgO

So, 4 moles of Mg give 4 moles of MgO.

Part (b): Moles of O₂ required

From the equation: 2 moles of Mg required O₂ = 1 mole

1 mole of Mg required O₂ = 12 mole

4 moles of Mg required O₂ = 12 × 4 = 2 moles of O₂

Hence, 4 moles of Mg needed 2 moles of O₂.

Steps for Mole-Mole Calculations

• Write the balanced chemical equation.
• Identify the known and unknown substances.
• Use the mole ratio from the coefficients.
• Calculate the moles of the unknown.

Intermediate Level

Mole Ratio from Balanced Equation

For the reaction: 2Mg + O₂ → 2MgO

Example 4.5

**Given:** - Moles of Mg = 4

• Moles of MgO formed
• Moles of O₂ required

Equation: 2Mg + O₂ → 2MgO

Solution:

Moles of MgO: 4 mol Mg × 2 mol MgO2 mol Mg = 4 mol MgO

Moles of O₂: 4 mol Mg × 1 mol O₂2 mol Mg = 2 mol O₂

• MgO formed = **4 moles**
• O₂ required = **2 moles**

Formula

Moles of unknown = Moles of known × Coefficient of unknownCoefficient of known

Important Points

• 1. Coefficients in a balanced equation = moles.
• 2. Mole ratio from coefficients.
• 3. Moles of unknown = moles of known × (coefficient of unknown / coefficient of known).
• 4. Example: 2Mg + O₂ → 2MgO.
• 5. 4 moles Mg → 4 moles MgO; 2 moles O₂.

Remember

• - Balanced equation: Coefficients = moles
• - Mole ratio: Coefficient of unknown / coefficient of known
• - Formula: Moles of unknown = moles of known × mole ratio
• - Example: 2Mg + O₂ → 2MgO

Common Mistakes

• 1. Forgetting to balance the equation: Always balance first.
• 2. Using the wrong mole ratio: Use the correct coefficients.
• 3. Mixing up reactants and products: Check the equation carefully.
• 4. Forgetting units: Moles are the unit.

Quick Revision

• Coefficients in a balanced equation = moles.
• Mole ratio from coefficients.
• Moles of unknown = moles of known × (coefficient of unknown / coefficient of known).
• Example: 2Mg + O₂ → 2MgO.
• 4 moles Mg → 4 moles MgO; 2 moles O₂.

60-Second Revision

• Balanced equation: Coefficients = moles
• Mole ratio: Coefficient of unknown / coefficient of known
• Formula: Moles of unknown = moles of known × mole ratio
• Example: 2Mg + O₂ → 2MgO

QUALITY VALIDATION

• Topic: 4.1.4 Mole-Mole Calculation
• Chapter: 4 (Stoichiometry)

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