Free Science Lessons: Using Moles to Calculate Masses in Chemical Reactions

Overview

• Objective: Use moles to calculate the masses of reactants and products in chemical reactions.
• Target Audience: Higher tier students.
• Structure: The topic is split over several videos.

Key Concepts

Moles Calculation

• Equation for Elements:
  • [ \text{Number of moles} = \frac{\text{Mass of chemical}}{\text{Relative atomic mass}} ]
• Equation for Compounds:
  • [ \text{Number of moles} = \frac{\text{Mass}}{\text{Relative formula mass}} ]
• Note: If unable to calculate relative formula mass, review the prerequisite video.

Applying Moles to Calculate Masses

Example Problem

• Problem: Calculate the mass of magnesium chloride from 72g of magnesium (chlorine is unlimited).
  1. Chemical Equation Setup:
     • Reactants: Magnesium (Mg), Chlorine (Cl)
     • Products: Magnesium Chloride (MgCl₂)
  2. Understanding Equation:
     • 1 mole of Mg = 1 mole of MgCl₂ (no large numbers mean '1')
  3. Calculate Moles of Magnesium:
     • [ 72 \text{ g (mass)} / 24 \text{ (relative atomic mass)} = 3 \text{ moles of Mg} ]
  4. Relate to Product:
     • 3 moles of Mg = 3 moles of MgCl₂
  5. Calculate Mass of MgCl₂:
     • [ \text{Mass} = \text{Number of moles} \times \text{Relative formula mass (95)} ]
     • [ 3 \times 95 = 285 \text{ g of MgCl₂} ]

Practice Problem 1

• Problem: Mass of calcium sulfate from 80g of calcium (sulfuric acid unlimited).
  • Steps:
    1. Calculate moles of calcium: [ 80 \text{ g} / 40 \text{ (relative atomic mass)} = 2 \text{ moles} ]
    2. Relate moles to product: 2 moles of Calcium = 2 moles of Calcium Sulfate
    3. Calculate mass: [ 2 \times 136 \text{ (relative formula mass)} = 272 \text{ g} ]

Practice Problem 2

• Problem: Mass of calcium carbonate needed to produce 224g of calcium oxide.
  • Steps:
    1. Calculate moles of calcium oxide: [ 224 \text{ g} / 56 \text{ (relative formula mass)} = 4 \text{ moles} ]
    2. Relate moles: 4 moles of Calcium Oxide = 4 moles of Calcium Carbonate
    3. Calculate mass: [ 4 \times 100 \text{ (relative formula mass)} = 400 \text{ g} ]

Additional Notes

• For more practice questions, refer to the revision workbook available through the provided link.
• The next video will cover more complex examples of calculating reacting masses.