Calculations Involving Colligative Properties

1
Calculations Involving Colligative Properties

• Chapter 16 Section 4

2
Review

• Molarity (M) moles of solute
  liter of solution
• Dilutions
• M1 x V1 M2 x 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
  solutes identity.
• Three Important Colligative Properties of
  Solutions.
• Vapor-pressure lowering
• Boiling-point elevation
• Freezing-point depression
• 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 Kf m 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 (p. 494)
• Depends upon the solvent
• Units (C/m)
• m Molal Concentration
• i Number of Ions from each formula unit

7
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 (p. 495)
• Depends upon the solvent
• Units (C/m)
• m Molal Concentration
• i Number of Ions from each formula unit

8
Molar mass from F-p D or B-p E

• g/mol (Kf)(g solute)(i)
• (?Tf)(kg solvent)
• g solute (?Tb)(kg solvent)(g/mol solute)
• (kb)(i)