Lecture: Concentration of Solutions

Introduction

• Presenter: Andre
• Topic: Main ways of expressing concentration of solutions
• Structure: Different units of concentration and examples

Molarity (M)

• Definition: Moles of solute over liters of solution
• Components:
  • Solute: Substance being dissolved (e.g., salt in a saltwater solution)
  • Solvent: Substance doing the dissolving (e.g., water in a saltwater solution)
• Example Problem:
  • Given a solution that evaporates from 0.15L to 0.105L, find the new concentration
  • Use formula: ( M_1 V_1 = M_2 V_2 )
  • Calculation: ( M_2 = \frac{M_1 \times V_1}{V_2} = 0.794 \text{ M} )
• Key Point: Concentration increases as solvent evaporates

Molality (m)

• Definition: Moles of solute over kilograms of solvent
• Example Problem:
  • Given 571.4g of ( H_2SO_4 ) and density 1.329g/cm³
  • Convert volume units: 1 cm³ = 1 mL, 1000 cm³ = 1L
  • Calculate mass of solution and solvent:
    • Solution mass: 1329g (from density)
    • Solvent mass: 1329g - 571.4g = 757.6g = 0.7576 kg
  • Calculate moles of solute using molar mass: ( 5.83 \text{ moles} )
  • Molality: ( 7.69 \text{ m} )

Mole Fraction

• Definition: Moles of solute over total moles of solution
• Example Problem:
  • Mixing 25g of water and 25g of ethanol
  • Calculate moles for each: ( 1.388 \text{ moles of } H_2O ), ( 0.543 \text{ moles of ethanol} )
  • Total moles: 1.931
  • Mole fraction:
    • Water: ( \frac{1.388}{1.931} = 0.719 )
    • Ethanol: ( \frac{0.543}{1.931} = 0.281 )

Percent by Mass

• Definition: Mass of solute over mass of solution (in grams) multiplied by 100%
• Example Problem:
  • Solution with 36.5g NaCl and total mass 3.55g
  • Calculation: ( \frac{36.5}{3.55} \times 100% = 10.3% )

Percent by Volume

• Definition: Volume of solute over volume of solution multiplied by 100%
• Example Problem:
  • 25 mL of ethanol mixed to make 200 mL total solution
  • Calculation: ( \frac{25}{200} \times 100% = 12.5% )

Conclusion

• Overview of various concentration expressions: molarity, molality, mole fraction, percent by mass, and percent by volume
• Encouragement to watch the next lecture on equilibrium and aqueous solutions

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