Colligative properties
Solubility
1
Solubility
1. What does the phrase “like dissolves like” mean.
2. Why does the solubility of alcohols decrease with increased carbon chain
length?
Alcohol
Methanol
Ethanol
Propanol
Butanol
Pentanol
Hexanol
Heptanol
Solubility in water (mol/100g water)
∞
∞
∞
0.11
0.030
0.0058
0.0008
3. How does temperature affect solubility of solids? Of gases?
4. How does pressure affect solubility of solids? Of gases?
5. Look up the structures of vitamins A,B,C,D,E and K. Why are vitamins B and
C water soluble and vitamins A,D,E and K are only soluble in fatty tissue. Why
can taking too much supplements of vitamins A,D,E and K be harmful?
DUE: Monday, December 16
(you can discuss together, but each person should write their own answer.)
Colligative properties
Solubility
2
Concentration
1.A solution is made by dissolving 13.5 grams glucose (C6H12O6) in 0.100 kG of
water. What is the mass percentage of solute in this solution?
2. A 2.5 grams sample of ground water was found to contain 5,4 µg of Zn2+.
What is the concentration of Zn2+ in ppm?
3. A commercial bleach contains 3.62 percent (by mass) sodium Hypochlorite,
NaOCl. What is the mass of NaOCl in a bottle containing 2500 g bleaching
solution?
4. A solution of HCl contains 36 % HCl ny mass. Calculate the mole fraction of
Hcl in solution. Calculate the molality of the HCl.
5. Given the density of a solution of 5.0 grams tolulene(C7H8) and 225 g benzene
is 0.876 g/ml calculate
a. molality
b. molarity
c. mass percent of solute
Solutions
Colligative properties
Solubility
3
1. Draw a diagram showing how NaCl dissolves
2. There are three things that will determine if a substance will dissolve and if
it does, whether or not it does so endo or exothermically.
3. Why do polar solvents dissolve polar solutes and nonpolar solvents can’t
dissolve polar solutes.
Example NaCl will dissolve in water but not gasoline
4. Is dissolving a chemical or physical change?
Look at Ni in HCl
and NaCl in H2O
5. Ways of measuring concentration
a.
Colligative properties
Solubility
4
b.
c.
1.
2.
3.
d.
e.
f
6. To convert between molarity and molality I need to know density of the
solution. Explain to me how I can do this conversion. Give an example.
Colligative Problems
Colligative property: properties that depend on the quantity (concentration) of
solute particles.
Vapor Pressure Lowering
What is vapor pressure?
Example
Explain the following situation.
Rauolt’s Law
PA = XAPA°
Example:Glycerin, C3H8O3, is a nonvolatile nonelectrolyte with a
density of 1.26 g/ml at 25°C. Calculate the vapor pressure at 25°C
of a solution made by adding 50.0 mL of glycerin to 500.0 mL of
water. The vapor pressure of pure water at 25°C is 23.8 torr.
Ans: 23.2 torr
Colligative properties
Solubility
5
Note: just like ideal gases follow ideal gas laws at high temp and low
pressure, Rauolt’s law is based on ideal solutions. An ideal solution is one
that have low concentrations and the solute and solvent molecules are
similar in size and have similar intermolecular forces.
If intermolecular forces between solvent and solute are weaker than
between solvent and solvent and between solute and solute, than the
vapor pressure tends to be greater than predicted by Raoult’s law.
Conversely, if the intermolecular forces between solute and solvent are
greater, the vapor pressure will be lower than predicted by Raoult’s law.
WHY?
Boiling point elevation
1.Why will adding solute to a solvent increase boiling point?
2. What will be its effect on a phase diagram?
3. ΔTb = Kb m I
Solvent
Water
Benzene
Ethanol
Carbon
tetrachloride
Chloroform
(CHCl3)
Normal bp
(°C)
100.0
80.1
78.4
76.8
Kb(°C/m)
Kf (°C/m)
0.52
2.53
1.22
5.02
Normal fp
(°C)
0.0
5.5
-114.6
-22.3
61.2
3.63
-63.5
4.68
1.86
5.12
1.99
29.8
Example: Automotive antifreeze consists of ethylene glycol, (C2H6O2) a
nonvolatile nonelectrolyte. Calculate the boiling point and freezing point of
a 25.0% mass solution of ethylene glycol in water.
(ans: bp:102.8°C; fp: -10.0°C)
Colligative properties
Solubility
6
Freezing point depression
Why does adding solute decrease the freezing point?
What will be its effect on the phase diagram?
ΔTb = Kb m I
Example: List the following aqueous solutions in order of their expected
freezing points: 0.050 m CaCl2; 0.15 m NaCl; 0.10m HCl; 0.050 m
HC2H3O2; o.10 m C12H22O11
Osmotic Pressure
The pressure required to prevent osmosis is the osmotic
pressure denoted by π.
π = (n/V)RT = MRT where M is the molarity of the solution. Use
the R constant to obtain the pressure in whatever units you choose.
isotonic Two solutions of identical osmotic pressure, no osmosis
will occur
hypotonic: if one solution is of lower osmotic pressure, it is
hypotonic with the more concentrated solution
hypertonic the more concentrated solution is said to be hypertonic
with respect to the dilute.
2 . Example: What is the osmotic pressure at 20°C of a 0.0020 M
sucrose(C12H22O11) solution? Ans: 0.048 atm OR 37 torr.
Colligative properties
Solubility
7
Determining Molar Mass
A solution of an unknown nonvolatile nonelectrolyte was
prepared by dissolving 0.250g in 40.0g CCl4. The normal
boiling point of the resultant solution was increased by
0.357 °C. Calculate the molar mass of the solute.
The osmotic pressure of an aqueous solution of a certain
protein was measured in order to determine its molar
mass. The solution contained 3.50 mg of protein dissolved
in sufficient water to form 5.0 mL of solution. The osmotic
pressure at 25°C was 1.54 torr. Calculate the molar mass
of the protein.