Topic: Colligative Properties
Adding a non-volatile (doesn’t readily
evaporate) solute affects…
•
•
•
•
Conductivity (electrolytes)
Freezing point
Boiling point
Vapor pressure
Freezing Point Depression
• Adding solute will lower the freezing point
• Which one will lower the freezing point more?
– NaCl
or
MgCl2
– Na+1 (aq) + Cl-1(aq)
Mg+2 (aq) + 2Cl-1(aq)
– 1 mol + 1 mol = 2 moles
1 mol + 2 mol = 3 moles
Vapor Pressure Decreased
• Solute particles at the surface get in the way
of some solvent molecules evaporating
Boiling Point Elevation
• Adding solute will raise the boiling point
• Which one will raise the boiling point more?
– Sugar (C12H22O11)
– C12H22O11 (aq)
– 1 mol
or
salt (NaCl)
Na+1 (aq) + Cl-1(aq)
1 mol + 1mol = 2 moles
Boiling Point
Freezing Point
Vapor Pressure
Depends on the # of moles
dissolved in the solution and NOT
on the type of particles
The higher the concentration of
solute in solvent, the more MP, BP,
and VP are affect
Which solution containing 1 mole of solute dissolved
in 1000 g of water has the lowest freezing point?
1) C2H5OH(aq)
2) NaCl(aq)
3) KOH(aq)
4) CaCl2(aq)
Be careful! What if
the question asked
which solution has
the highest
freezing point?
C6H12O6
• Covalent
• Dissolves as molecules
C6H12O6(s)  C6H12O6(aq)
• 1 mole of sugar yields 1 mole of
molecules
NaCl
• Ionic
• Dissolves as ions
• NaCl(s)  Na+(aq) + Cl-(aq)
• 1 mole of salt yields 2 moles of ions.
Get more particles from salt than
sugar.
MgCl2
• Ionic
• Dissolves as ions
• MgCl2(s)  Mg2+(aq) + 2Cl-(aq)
• 1 mole of salt yields 3 moles of ions