Understanding Percentage Composition in Chemistry

Key Concepts:

• Percentage Composition: Commonly tested from grades 10 to 12 in stochiometry and quantitative aspects of chemical change. Also referred to as percentage by mass.
• Example Explanation: Bag of Substances
  • If a bag has 30g rice and 40g beans, total mass = 70g.
  • To find percentage of rice:
    • Formula: [(\frac{Mass, of, rice}{Total, mass}) \times 100]
    • Calculation: [(\frac{30g}{70g}) \times 100 = 42.86%]
  • Similarly, for beans: [(\frac{40g}{70g}) \times 100 = 57.14%]

Steps to Calculate Percentage Composition:

1. Calculate the total molar mass of the compound
   • Utilize atomic mass numbers from the periodic table.
2. Determine the mass of each element within the compound
3. Apply the formula:
   • Formula: [(\frac{Mass, of, specific, element}{Total, molar, mass, of, compound}) \times 100]
   • This formula will help calculate the percentage composition for each element.

Example 1: Water (H2O)

• Step 1: Calculate Molar Mass
  • Hydrogen: ((1 \times 2) = 2)
  • Oxygen: ((16 \times 1) = 16)
  • Total mass: (2 + 16 = 18g/mol)
• Step 2: Calculate Individual Element Mass
  • Hydrogen: (\frac{2}{18} \times 100 = 11.11%)
  • Oxygen: (\frac{16}{18} \times 100 = 88.89%)

Example 2: Magnesium Hydroxide (Mg(OH)2)

• Identify Elements: Magnesium (Mg), Oxygen (O), Hydrogen (H)
• Step 1: Calculate Molar Mass
  • Magnesium: (24)
  • Oxygen: ((16 \times 2) = 32)
  • Hydrogen: ((1 \times 2) = 2)
  • Total mass: (24 + 32 + 2 = 58g/mol)
• Step 2: Calculate Individual Element Mass
  • Magnesium: (\frac{24}{58} \times 100 = 41.38%)
  • Oxygen: (\frac{32}{58} \times 100 = 55.17%)
  • Hydrogen: (\frac{2}{58} \times 100 = 3.45%)

Challenging Example: Ammonium Nitrate (NH4NO3)

• Calculate the mass of nitrogen in a 30 kg bag:
  • Step 1: Calculate Molar Mass
    • Elements: Two Nitrogens (N), Four Hydrogens (H), Three Oxygens (O)
    • Nitrogen: ((2 \times 14) = 28)
    • Hydrogen: ((4 \times 1) = 4)
    • Oxygen: ((16 \times 3) = 48)
    • Total mass: (28 + 4 + 48 = 80g/mol)
  • Step 2: Calculate Percentage of Nitrogen
    • (\frac{28}{80} \times 100 = 35%)
  • Step 3: Calculate Mass of Nitrogen in 30 kg Bag
    • (30 \times 0.35 = 10.5 kg)
• Note: Only the percentage of nitrogen was calculated because the question specifically asked for the mass of nitrogen.

Final Tips

• Always cross-check the sum of the individual percentages to ensure they approximate 100%.
• When encountering complex compounds, break down the elements clearly and apply the distributive law where applicable.