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Calculations Involving Colligative Properties

Calculations Involving Colligative Properties. Review. Molarity (M) = moles of solute liter of solution Dilutions: M 1 x V 1 = M 2 x V 2 Percent by volume (%(v/v)) = volume of solute x 100% volume of solution

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Calculations Involving Colligative Properties

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  1. Calculations Involving Colligative Properties

  2. Review Molarity (M) = moles of solute liter of solution Dilutions: M1x V1= M2x V2 Percent by volume (%(v/v)) = volume of solute x 100% volume of solution Percent by mass (%(m/m)) = mass of solute x 100% mass of solution

  3. Review • Colligative Property: A property that depends only upon the solute concentration and not on the solute’s identity. • Four Important Colligative Properties of Solutions. • Vapor-pressure lowering • Boiling-point elevation • Freezing-point depression • Vapor Pressure Molarity (M) = moles of solute liter of solution Molality (m) = moles of solute_ kilogram of solvent • Molality is the number of moles of solute dissolved in 1 kilogram (1000 g) of solvent.

  4. Mole Fraction (another way to express concentration) Mole Fraction: the ratio of the moles of the solute to the total number of moles of solvent and solute. XA= nA___ XB= nB___ nA + nB nA + nB

  5. Freezing-Point Depression & Boiling-Point Elevation

  6. Freezing-Point Depression ∆Tf = Kfm i • ∆Tf = Freezing Point Depression • Difference between the freezing point of the solution and the freezing point of pure solvent • Kf = Molal Freezing Point Depression Constant • Equals to the change in freezing point for a 1-molal solution of a nonvolatile molecule solid • Values are listed in a table • Depends upon the solvent • Units (°C/m) • m = Molal Concentration • i = Number of Ions from each formula unit

  7. Freezing Point Depression

  8. Applications salting icy roads making ice cream antifreeze cars (-64°C to 136°C) fish & insects

  9. Boiling-Point Elevation ∆Tb = Kbm i • ∆Tb = Boiling-Point Elevation • Difference between the boiling point of the solution and the boiling point of pure solvent • Kb = Molal Boiling Point Elevation Constant • Equals to the change in boiling point for a 1-molal solution of a nonvolatile molecule solid • Values are listed in a table • Depends upon the solvent • Units (°C/m) • m = Molal Concentration • i = Number of Ions from each formula unit

  10. Solution Calculations • What is the molarity of a 1.11 ppm solution of Zn2+ ions?

  11. How Vapor Pressure Depression Occurs • Solute particles take up space in a solution. • Solute particles on surface decrease # of solvent particles on the surface. • Less solvent particles can evaporate which lowers the vapor pressure of a liquid.

  12. Vapor Pressures of Pure Water and a Water Solution The vapor pressure of water over pure water is greater than the vapor pressure of water over an aqueous solution containing a nonvolatile solute. Solute particles take up surface area and lower the vapor pressure

  13. Raoult’s Law • Vapor pressure of a solution varies directly as the mole fraction of solvent Vapor PressureSolution = (molefractionsolvent)(Vapor Pressuresolvent)

  14. Solid Calculations • Chemical analysis showed 1.23 mg Fe in a 15.67 g sample of soil. • What is the Fe concentration in ppm?

  15. Unusual concentration units • How many nano moles of Cu are present in 12.3 µL of 25 ppm CuSO4?

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