(40) GCSE Chemistry Revision "Calculating Percentage by Mass"

Understanding Percentage by Mass

Concept Overview

• The percentage by mass of an element in a compound tells us what portion of the compound's mass comes from that particular element.

• To calculate this, we find the ratio of the element’s total atomic mass to the compound's total relative formula mass, then multiply by 100.

Key Terms

• Relative Formula Mass (Mr): The sum of the relative atomic masses of all atoms in the compound.

• Relative Atomic Mass: Indicates the mass of one atom of a specific element compared to carbon-12.

Example 1: Magnesium Oxide (MgO)

Analyzing the Compound

• Formula: MgO (contains magnesium and oxygen)

• Atoms: 1 atom of Magnesium (Mg) and 1 atom of Oxygen (O).

Calculation of Relative Formula Mass

• Relative Atomic Mass of Magnesium: 24

• Relative Atomic Mass of Oxygen: 16

• Relative Formula Mass of MgO = 24 (Mg) + 16 (O) = 40

Calculating Percentage by Mass for Magnesium

• Formula: ( \text{Percentage by Mass of Magnesium} = \frac{\text{Relative Atomic Mass of Mg}}{\text{Relative Formula Mass of MgO}} \times 100 )

• Calculation: ( \frac{24}{40} \times 100 = 60% )

• Conclusion: Percentage by mass of Magnesium in MgO is 60%.

Example 2: Calcium in Calcium Chloride (CaCl2)

Analyzing the Compound

• Formula: CaCl2 (contains calcium and chlorine)

• Atoms: 1 atom of Calcium (Ca) and 2 atoms of Chlorine (Cl).

Calculation of Relative Formula Mass

• Relative Atomic Mass of Calcium: 40

• Relative Formula Mass of CaCl2 = 40 (Ca) + 2*35.5 (Cl) = 111

Calculating Percentage by Mass for Calcium

• Formula: ( \text{Percentage by Mass of Calcium} = \frac{40}{111} \times 100 )

• Calculation: ( \frac{40}{111} \times 100 \approx 36% )

• Conclusion: Percentage by mass of Calcium in CaCl2 is 36%.

Example 3: Hydrogen in Methane (CH4)

Analyzing the Compound

• Formula: CH4 (contains carbon and hydrogen)

• Atoms: 1 atom of Carbon (C) and 4 atoms of Hydrogen (H).

Calculation of Relative Formula Mass

• Relative Atomic Mass of Hydrogen: 1

• Total Atomic Mass for Hydrogen in CH4: 4*1 = 4

• Relative Formula Mass of CH4 = 12 (C) + 4 (H) = 16

Calculating Percentage by Mass for Hydrogen

• Formula: ( \text{Percentage by Mass of Hydrogen} = \frac{4}{16} \times 100 )

• Calculation: ( \frac{4}{16} \times 100 = 25% )

• Conclusion: Percentage by mass of Hydrogen in CH4 is 25%.

Example 4: Sodium in Sodium Sulfate (Na2SO4)

Analyzing the Compound

• Formula: Na2SO4 (contains sodium, sulfur, and oxygen)

• Atoms: 2 atoms of Sodium (Na), 1 atom of Sulfur (S), and 4 atoms of Oxygen (O).

Calculation of Relative Formula Mass

• Relative Atomic Mass of Sodium: 23

• Total Atomic Mass for Sodium in Na2SO4: 2*23 = 46

• Relative Formula Mass of Na2SO4 = 46 (Na) + 32 (S) + 64 (O) = 142

Calculating Percentage by Mass for Sodium

• Formula: ( \text{Percentage by Mass of Sodium} = \frac{46}{142} \times 100 )

• Calculation: ( \frac{46}{142} \times 100 \approx 32.4% )

• Conclusion: Percentage by mass of Sodium in Na2SO4 is 32.4%.