1. Properties of Solutions
Chapter 11
Properties of Solutions

2. Solution Composition Molarity (M) = moles solute / Liters solution
Mass Percent – no units
(mass solute / mass solution) x 100
Mole Fraction – no units – decimal number
χ a= (mol a / mol total)
Molality (m) = moles solute / kg solvent
Does not change with temp

3. Example - #25 p. 548
A solution is made by dissolving 25g of NaCl in enough water to make 1.00L of solution. Calculate the mass percent, molarity, molality, and mole fraction of the sodium chloride. Assume the density of the solution is 1.00g/mL

4. Solubility Likes dissolve likes For solids Gases Polar dissolves polar
Nonpolar dissolves nonpolar
For solids
Increased Temp usually increases solubility
Gases
Increased Temp decreases solubility
Increased Pressure increases solubility
Henry’s Law

5. Henry’s Law
The amount of gas dissolved in a solution is directly proportional to the pressure of the gas above the solution
P=kC
P is the pressure above the gas above the solution
k is constant for a solution
C is the concentration of the dissolved gas

9. Implications of Gas Solubility
From:

10. Vapor Pressure of Solutions
A nonvolatile solute added to a solvent ALWAYS lowers the vapor pressure of the solution
Fewer molecules to enter the gas phase
Follows Raoult’s Law
Called Ideal solutions

11. Raoult’s Law Psolution = χsolvent * Psolvent Where
Psoluiton = Vapor Pressure of Solution
χsolvent = mole fraction of solvent
Psolvent = Vapor Pressure of the solvent
Since the mole fraction is always less than one the pressure of the solution is lowered

12. Example - #45 p. 549
A solution is prepared by mixing 50.0 grams of glucose C6H12O6 with g of water. What is the vapor pressure of the solution at 25ºC? The vapor pressure of water at 25ºC is 23.8 torr.

13. More on Raoult’s Law
Nonelectrolyes (covalent compounds) have the expected change you calculate
Electrolytes (Ionic compounds) will see a bigger change in vapor pressure
Ex. Ethanol lowers vapor pressure of water as expected
NaCl doubles the lowering of the vapor pressure. MgCl2 triples

14. Deviations From Ideal
When the solute and the solvent have similar polarity the vapor pressure of the solution is lower
Attraction b/t particles
Negative deviation
When the solute and the solvent have dissimilar polarity the vapor pressure of the solution is higher
Positive deviation

15. Effect of Solute on Solution
What effect will the lowering of the vapor pressure have on the solution?
Boiling point will go up
Freezing point will go down

16. Colligative Properties
Properties that depend on the number of particles in solution
Vapor Pressure Depression
Boiling Point Elevation
Freezing Point Depression
Osmotic Pressure

17. Boiling Point Elevation
ΔTb = Change in the boiling point
Kb = Boiling point elevation constant Units of (ºC*kg/mol)
Constant for a given substance
Data found in table 11.5 on page 532
If ΔTb for salt water is 2.2ºC then the new B.P. is 102.2

18. The van’t Hoff Factor i = The van’t Hoff factor
Tells you how many moles of particles a solute would form if 1 mole dissolves
All covalent compounds have an i of 1
They do not ionize
i for ionic compounds equals the # of ions
NaCl = 2
MgCl2 = 3
AlCl3 = 4

19. Freezing Point Depression
ΔTf = Change in the freezing point
Kf = Freezing point depression constant Units of (ºC*kg/mol)
If ΔTf for salt water is 2.2ºC then the new F.P. is -2.2ºC

20. Supercooling
From:

22. Example
What is the freezing point of water if 0.25 mol. of glucose is dissolved in 500mL of water? Kf=1.86 ºC*kg/mol?

23. Example
What is the new boiling point of water when 100. grams of NaCl is dissolved in 2.0L of water? Kb=0.51 ºC*kg/mol

24. Example
Ethylene Glycol (C2H6O g/mol) is placed in radiators to prevent the water from boiling. How many moles of EG must be added to 10.0L of water to make the water boil at ºC? Kb=0.51 ºC*kg/mol

25. Osmosis
The flow of solvent molecules into a solution through a semipermeable membrane
High concentration of solvent to low concentration of solvent

26. Osmotic Pressure The Pressure required to stop osmosis
Pressure required to cause reverse osmosis
Π=iMRT
Π = osmotic pressure (atm)
i = van’t Hoff factor
M = Molarity
R = L*atm/mol*K
T = Temp in K

27. Homework
P.548 #’s 26,28abd,38,46,57,63,70,72