Bell Work: Molarity
1. Moles of solute divided by liters of solution is
called _________.
2. Moles of solute divided by mass of solution is
called _________.
3. The ratio to convert grams to moles is 1 ____
divided by the ____ ____.
4. The ratio to convert moles to grams is the ____
____ divided by 1 ____.
5. Molarity measure ( diluteness / temperature /
concentration ) of a solution.
Colligative
Properties
Colligative Properties
Colligatiave properties are:
•physical properties
•influenced by the number or amount of
solute particles but NOT by the identity
of those particles.
Analogy: A gold-digger is a person
who only cares about the amount of
money a person has not who the
person is. This is the same with
colligative properties.
There are three colligative
properties that change when the
amount of solute is changed.
vapor pressure lowering
boiling point elevation
freezing point depression
Vapor Pressure Lowering
• Pressure exerted in a
closed container (like a
soda bottle with the top on)
by the solvent particles as
they evaporate to form a
gas.
• The vapor exerts pressure
on the container and the
surface of the liquid.
• More solute, less vapor
pressure.
• They are inversely related.
Boiling Point Elevation
•When the vapor pressure (pushing up)
equals atmospheric pressure (pushing
down), the solvent boils.
•If the vapor pressure is lowered by the
solute, it has to get hotter to make
enough vapor to boil.
•So the more solute, the more
the boiling point is elevated.
•They are directly related.
Freezing Point Depression
•Temperature is a measure of
the average kinetic energy of
particles.
• Freezing is the point when
solvent particles get very close
together because they do not
have enough kinetic energy to
stay far apart so they are
pulled together to solidify.
kinetic energy of solids
• The solute particles get in
the way of the formation
of the solid so the more
solute, the lower the
freezing point.
• They are inversely related.
How do the number of solute
particles in a solution change and
affect these 3 physical properties?
• Solute particles INTERFERE with
the solvent particles!
• They act like a “third wheel” in the
way of everything.
• When a solute is added to a
solvent, solvation and dissociation
begin to take place.
•Only ionic compounds are
able to break apart as
they dissociate
•These compounds are
called electrolytes
because their aqueous
strong electrolyte
solutions can conduct an
electric current.
• Ionic solutes break apart
and always produce more
than one ion which makes
the number of solute
particles increase.
NaCl electrolyte
Remember: The more
solute particles that are in
the solution, the more the
colligative properties are
affected!
Let’s Summarize
• Ionic Compounds will dissociate
and produce electrolytes that
are capable of conducting an
electric current
• The following solutes are placed
into water. What happens to
these solutes as they begin to
dissolve?
They dissociate into:
strong electrolyte
• NaCl  1 Na+1 + 1 Cl-1 2 moles of
ions
• MgCl2  1 Mg+2 + 2 Cl-1 3 moles of
ions
• Na3PO4  3 Na+1 + 1 PO4-3
4 moles of
ions
Q: Which substance above produces
more solute particles in solution?
A: Na3PO4 ; therefore, it will have
the most influence on colligative
properties
***Remember: A colligative
property depends on the number of
solute particles in solution, not the
identity of the particles
•Covalent solutes dissolve, but
they do NOT dissociate into
ions. Why?
•They do NOT have regions of
opposite charge to attract to
the nonpolar solvent!
• Instead, covalent
substances dissolve as
whole molecules.
• The whole molecules
form nonelectrolytes
and they DO NOT
conduct an electric
current!
nonelectrolyte
nonelectrolyte
The solutes below are placed into
water. What happens to these
solutes as they begin to dissolve?
C6H12O6  1 C6H12O6
1 mole of
particles
C12H22O11  1 C H O
12 22 11
1 mole of
particles
**They dissociate one molecule at a
time!
Q: Which covalent substance above
would have the most influence on
colligative properties?
A: Neither! They would have the
same effect b/c each covalent
compound produces only ONE mole of
solute particle and colligative
properties depend on the number of
particles!
Let’s Summarize
• Covalent Compounds will
dissociate into only one
mole of particles and
produce nonelectrolytes
that are NOT able to
conduct an electric current!
Now it’s your turn!
Why do ionic compounds more strongly
affect colligative properties than
covalent compounds?
Ionic compounds always dissociate into
two or more particles while covalent
compounds always dissociate into only
one! Since colligative properties
depend on the NUMBER of particles,
and ionic compounds produce more
particles, they affect colligative
properties more!
How does the addition of a solute affect
each of the 3 colligative properties?
Vapor pressure decreases with addition of
solute to solvent!
• There are solute particles interfering with
the solvent particles at the surface of the
solution.
• Less of the solvent can evaporate and push
upwards b/c the solute is at the surface, too.
• Since less solvent is evaporating, vapor
pressure drops b/c there is not gas pushing
upwards.
vapor pressure
Vapor Pressure of a Solution: A Colligative
Property
Which flask has a lower vapor pressure?
Explain.
Flask A
Pure Solvent
(no solute present)
Flask B
Solution
Solute + Solvent
How does the addition of a solute affect
each of the 3 colligative properties?
Boiling point elevates (increases) with
addition of a solute to a solvent.
• For a substance to boil, atmospheric
pressure pushing down must equal vapor
pressure pushing up.
• Solute particles interfere with the
solvent particles at the surface.
• It will take more energy to
get the solvent particles to
“push” their way to the
surface to evaporate.
• This means you have to heat
the solution up to a higher
temperature to give the
solvent particles more
energy to increase vapor
pressure.
How does the addition of a solute affect
each of the 3 colligative properties?
• Freezing point depresses
(decreases) with addition of solute
to solvent.
• Solute particles are mixed in with
the solvent particles interfering
with the ability of solvent particles
to “get close” together.
• The solute is “in the way.”
• This means temperature
has to drop A LOT for the
solvent to push the solute
out of the way in order to
get close together and
solidify.
Ionic Compounds
Covalent Compounds
polar (have + and – nonpolar (no oppositely
charged poles)
regions)
since no ions, they
dissociate into two or
dissociate one particle at a
more particles at a time
time
ions are called
electrolytes
conduct
electricity
no ions so they are
called non-electrolytes
strongly affect
colligative properties
more weakly affects
colligative properties
non-conductors of
electricity