Free Science Lessons: Calculating Percentage by Mass

Objective

• Learn to calculate the percentage by mass of an element in a compound.

Key Concepts

Magnesium Oxide Example

• Formula: MgO
• Elements: Magnesium (Mg) and Oxygen (O)
• Atoms: 1 Mg, 1 O
• Relative Atomic Masses (Ar):
  • Magnesium (Mg) = 24
  • Oxygen (O) = 16
• Relative Formula Mass (Mr): 24 + 16 = 40

Calculating Percentage by Mass

• Formula: [ \text{Percentage by mass of an element} = \frac{\text{Total relative atomic mass of element}}{\text{Relative formula mass of compound}} \times 100 ]

Example Calculations

Magnesium in Magnesium Oxide

• Calculation:
  • Total Ar of Mg = 24
  • Mr of MgO = 40
  • Percentage by mass = ( \frac{24}{40} \times 100 = 60% )

Calcium in Calcium Chloride

• Formula: CaCl₂
• Mr: 111
• Ar of Calcium: 40
• Calculation:
  • Total Ar of Ca = 40
  • Percentage by mass = ( \frac{40}{111} \times 100 \approx 36% )

Hydrogen in Methane

• Formula: CH₄
• Mr: 16
• Ar of Hydrogen: 1
• Calculation:
  • Total Ar of Hydrogen = 4 (since there are 4 H atoms)
  • Percentage by mass = ( \frac{4}{16} \times 100 = 25% )

Sodium in Sodium Sulfate

• Formula: Na₂SO₄
• Mr: 142
• Ar of Sodium: 23
• Calculation:
  • Total Ar of Na = 46 (since there are 2 Na atoms)
  • Percentage by mass = ( \frac{46}{142} \times 100 = 32.4% )

Additional Resources

• For more practice questions, refer to the revision workbook mentioned in the video.

This summary captures the essential steps and formulas needed to calculate the percentage by mass of an element in various compounds, providing practical examples for understanding.