Lecture: Calculating Percentage Purity

Introduction

• Percentage Purity: Defined as the percentage of a pure compound in an impure sample.
• Formula: [ \text{Percentage Purity} = \left( \frac{\text{Mass of Pure Compound}}{\text{Total Mass of Impure Sample}} \right) \times 100 ]

Example 1: Copper Ore

• Problem: Calculate the percentage purity of a 150 gram copper ore sample containing 87.3 grams of pure copper.
• Solution:
  • [ \frac{87.3}{150} \times 100 = 58.2% ]

Example 2: Hydrated Ethane Dioic Acid

• Problem: 5 grams of an impure sample dissolved in water to make 1 dm³ solution, titrated against NaOH.
• Balanced Equation:
  • 1 mole Ethane Dioic Acid reacts with 2 moles NaOH.
• Steps:
  1. Calculate Moles of NaOH:
     • Use ( N = C \times V )
     • ( C = 0.1 ) mol/dm³, ( V = 14 ) cm³ converted to 0.014 dm³.
     • ( N = 1.4 \times 10^{-3} ) moles.
  2. Calculate Moles of Ethane Dioic Acid:
     • Molar ratio 1:2, thus ( \frac{1.4 \times 10^{-3}}{2} = 7 \times 10^{-4} ) moles.
  3. Moles in 1 dm³ Solution:
     • ( \frac{7 \times 10^{-4} \times 1000}{25} = 2.8 \times 10^{-2} ) moles.
  4. Convert Moles to Mass:
     • Multiply by molar mass to get 3.53 grams.
  5. Calculate Percentage Purity:
     • ( \frac{3.53}{5} \times 100 = 70.6% )

Example 3: Impure Calcium Carbonate

• Problem: 10 grams of chalk reacted with excess HCl produces 2.13 dm³ CO₂ at STP.
• Reaction Equation:
  • 1 mole CaCO₃ reacts with 2 moles HCl to produce CO₂.
• Steps:
  1. Calculate Moles of CO₂:
     • ( \frac{2.13}{22.7} = 9.38 \times 10^{-2} ) moles (1:1 ratio with CaCO₃).
  2. Calculate Mass of CaCO₃:
     • ( 9.38 \times 10^{-2} \times 100.09 = 9.38 ) grams.
  3. Calculate Percentage Purity:
     • ( \frac{9.38}{10} \times 100 = 93.8% )