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Chem. 1A Colligative Properties Discussion 1
Colligative Properties Discussion
Chem. 1A
The material covered today is found in sections
Chapter 14.1 & 14.
This material will not be covered in lecture, you
will have homework assigned.
Chem. 1A Colligative Properties Discussion 2
Colligative Property
- Comparing physical properties of a solution relative to the physical properties of the pure solvent.
- Defined: Properties that depend on the total amount of dissolved solute particles in a solvent.
- More solute particles (ions or molecules) the greater this effect.
- Example: Solution: NaCl dissolved in water [Na +(aq) & Clโ(aq)] Pure solvent: water
Chem. 1A Colligative Properties Discussion 3
Colligative Properties
- Relative to a pure solvent, a solution has:
- A lower vapor pressure
- A higher boiling point
- A lower freezing point
- Osmosis occurs
- Example:
- Pure water: b.p.=100ยฐC f.p.= 0 ยฐC
- 1.00 m NaCl (^) (aq) b.p. = 101 ยฐC f.p. = โ3.7 ยฐC
Chem. 1A Colligative Properties Discussion 4
Concentration
- Colligative properties:
- Dependant upon the amount of solute particles.
- Concentration is expressed in different ways for different properties. (vapor pressure, boiling point, etc...)
Chem. 1A Colligative Properties Discussion 7
Concentration Units
X i =
moles solute (i)
Total moles in solution
Mole Fraction ( X )
Where โiโ is the moles of one component of the solute. Total moles are all species: mols solute (i) + mols solvent
โXi^ =^1
The sum of all of the mole fractions for each component are 1 exactly.
Chem. 1A Colligative Properties Discussion 8
Concentration Units
m =
moles solute
kg solvent
Molality ( m )
Chem. 1A Colligative Properties Discussion 9
Changes in vapor pressure: Raoultโs Law
- Vapor pressure: pressure exerted by the vapor when a liquid in a closed container is at equilibrium with the vapor.
- Vapor pressure of solution is decreased by the presence of a solute.
- More solute particles, lower vapor pressure of solution.
Chem. 1A Colligative Properties Discussion 10
Psolution = X (^) solvent ร Po
Psolution = the vapor pressure of a mixture of solute and solvent Po^ = the vapor pressure of the pure solvent X (^) solvent = the mole fraction of the solvent. ******
The expression can also be written in the form: o ฮP (^) solvent = โX (^) soluteรP
Raoultโs Law
ฮP is the change to the vapor pressure of the pure solvent.
Chem. 1A Colligative Properties Discussion 13
vanโt Hoff factor
BoilingBoiling andand FreezingFreezing pointpoint effectseffects involvinginvolving ions:ions: When solutions containing ions are involved, the total concentration of solute particles must be considered. The change in b.p. or f.p. is given by the equation: ฮT = K ร m รi Where m is the calculated molaity based on formula wt. โiโ = the number of ions (vanโt Hoff factor) compound Type i CH 3 OH molecular 1 NaCl strong electrolyte 2 Ba(NO 3 ) 2 strong electrolyte 3 HNO 2 weak electrolyte 1 โ 2
Chem. 1A Colligative Properties Discussion 14
A semipermeable membrane allows only the movement of solvent molecules.
Solvent Solution
Semipermeable membrane
Solvent molecules move from pure solvent to solution in an attempt to make both have the same concentration of solute.
Osmosis Osmosis
Chem. 1A Colligative Properties Discussion 15
OsmosisOsmosis (^) atat thethe ParticulateParticulate LevelLevel
Chem. 1A Colligative Properties Discussion 16
ProcessProcess ofof OsmosisOsmosis
