Lecture Notes: Volume Percentage in Solutions

Introduction to Mass and Volume Percentage

• Mass Percentage: Useful for making solutions with solid solutes.
  • Measure the mass of the solute without involving moles.
• Volume Percentage: Useful for solutions where components are liquids.
  • Easier to measure volume than mass for liquids.
  • Can convert volume to mass using density.

Example: Rubbing Alcohol

• Rubbing Alcohol (Isopropanol): Typically around 70% by volume.
• Volume Calculation for 300mL bottle:
  • Total solution volume = 355 mL.
  • Alcohol volume = 355 mL × 0.7 = 248.50 mL.
  • Density of Isopropanol = 0.785 g/mL.
  • Mass of Alcohol = 248.50 mL × 0.785 g/mL ≈ 195 grams.
  • Conclusion: This is a 70% alcohol by volume solution.

Comparing Concentrations

• Second Brand Example:
  • Advertised as 225g of isopropanol in a 400mL bottle.
  • Convert mass to volume using density:
    • Volume of Alcohol = 225g / 0.785 g/mL ≈ 274 mL.
  • Calculate Volume Percentage:
    • Volume Percentage = 274 mL / 400 mL × 100 ≈ 68.5%.
  • Conclusion: Slightly less concentrated than the first bottle.

Important Takeaways

• Volume Percentage: Key for liquid solutions and often more practical than measuring mass.
• Usage of Density: Crucial for converting between mass and volume for accurate solutions.