1. Colligative Properties

2. Does solute change the properties of a solvent?
Consider aqueous solutions
Solvent = water
How do properties of aqueous solution compare to properties of pure water?

3. Conductivity Pure water does not conduct electricity
Some aqueous solutions do
To conduct current, solution must contain ions
Conductivity depends on nature of the particle
Molecular (covalent) solutes do not conduct; ionic solutes do conduct

4. Colligative Properties
Depend on number particles in solution not on type particles
i.e. doesn’t matter what kind of particles: can be ions or molecules
Concentration of particles does matter

5. What are some colligative properties?
Vapor Pressure (↓)
Freezing Point (↓)
Boiling Point (↑)
change in Osmotic Pressure
higher the concentration of solute particles, the larger the effect

6. Does it matter if solute is ionic or covalent?
number of particles will vary
Ionic solutes
dissolve to produce (+) & (-) ions  more particles per mole of solute
Covalent solutes
1-to-1 relationship between moles of solute and moles of dissolved particles

7. C6H12O6 Covalent substances: C6H12O6(s) + H2O(l)  C6H12O6(aq)
dissolve as molecules:
C6H12O6(s) + H2O(l)  C6H12O6(aq)
1 mole sugar → 1 mole molecules

8. NaCl Ionic substances: NaCl(s) + H2O(l)  Na+1(aq) + Cl-1(aq)
dissolves (dissociates) as ions:
NaCl(s) + H2O(l)  Na+1(aq) + Cl-1(aq)
1 mole salt  2 moles ions

9. MgCl2 Ionic substance: 1 mole salt 3 moles ions
dissolves as ions
MgCl2(s) + H2O(l)  Mg2+(aq) + 2Cl-(aq)
1 mole salt 3 moles ions

10. As solute is added to a solvent, what happens to freezing point & boiling point of solution?
The freezing point ↓ & the boiling point ↓
The freezing point ↓ & the boiling point ↑
The freezing point ↑ & the boiling point ↓
The freezing point ↑ & the boiling point ↑
Answer: 2

11. - do you want greatest or least change?
Which solution containing 1 mole of solute dissolved in 1000 g of water has the lowest freezing point?
C2H5OH(aq)
NaCl(aq)
KOH
CaCl2
Be careful!
- do you want greatest or least change?
What if question asked which solution has highest freezing point?
Answer:

12. Of the following solutions, the one that will freeze at the lowest temperature contains 1 mole of nonvolatile solute dissolved in
1) 250 g of solvent
2) 500 g of solvent
3) 750 g of solvent
4) g of solvent
Answer: 1

13. Which solute, when added to 1000 g of water, will produce a solution with the highest boiling point?
1) 29 g of NaCl
2) 58 g of NaCl
3) 31 g of C2H6O2
4) 62 g of C2H6O2
1 mole of NaCl →
2 moles of ions
1 mole of C2H6O2 →
1 mole of molecules

14. Which solution will freeze at the lowest temperature?
1) 1 g of NaCl dissolved per 100 g of H2O
2) 1 g of NaCl dissolved per 1000 g of H2O
1 g of C12H22O11 dissolved per 100 g of H2O
4) 1 g of C12H22O11 dissolved per 1000 g of H2O

15. Which solution will freeze at the lowest temperature?
1) 1 g of NaCl dissolved per 100 g of H2O
__1 g____ = mol → mol of ions
58.8 g/mol (NaCl) (Na+1 and Cl-1)
2) 1 g of C12H22O11 dissolved per 100 g of H2O
__1 g___ = mol of molecules
232 g mol

16. Effect of Nonvolatile Solute
Boiling Point Elevation (↑)
Freezing Point Depression (↓)
The more particles, the bigger the effect!

17. Applications of colligative properties
Salting roads in the winter time
Using salted ice to make ice cream