1. Chapter 15
“Solutions”

2. Solution Definitions to Know & Use
Solution – homogenous mixture of two or more substances in a single physical state.
Solute – the substance being dissolved.
Solvent – the principal component that dissolves another component of a solution.
Solubility – a quantifiable measure of the degree to which a substance dissolves in another substance.
Soluble – a substance that can be dissolved in another substance.
Insoluble – a substance that cannot be dissolved in another substance.

3. Chapter 15 “Solutions” What are solutions?
Homogeneous mixtures of two or more substances in a single physical state.
A solution consists of a solute dissolved in a solvent.
Many examples exist.
What are the intrinsic properties of solutions?
Contain very small particles
(atoms, ions, molecules)
Homogeneous throughout. (Particles are evenly distributed on a molecular level).
Particles do not separate with time under constant conditions.
Diverse physical states and chemical compositions.

4. Types of Solutions Solid Solutions Gaseous Solutions Liquid Solutions
Alloys (14 carat gold, stainless steel, brass)
Gaseous Solutions
Air, scuba diving gases, vehicle exhaust
Liquid Solutions
Vinegar, antifreeze,
Aqueous Solutions
Solutions with water as solvent.
Seawater, soft drinks

5. 15-2 Concentration of Solutions
Concentration – the amount of solute dissolved per unit of solvent.
There are many ways to describe concentration, but they are either qualitative or quantitative.
Qualitative – a representation of the general nature of a solution.
Quantitative – a measure of the amount of a solute dissolved in the solution.

6. Qualitative Descriptions of Solutions
Dilute – a solution containing very little solute.
Concentrated – a solution containing a large amount of solute.

7. Qualitative Descriptions of Solutions
Saturated – a solution containing the maximum amount of solute that can be dissolved at the current temperature/pressure.
Unsaturated – a solution containing less than the maximum amount of solute that can be dissolved at the current temperature/pressure.
Supersaturated – an unstable condition in which a solution contains more than the maximum amount of solute that can normally be dissolved at the current temperature/pressure.

8. Solution Stability
Recall that a saturated solution contains the maximum amount of solute that can be dissolved at given conditions.
In a saturated solution, the rate of solute entering into solution precisely balances the rate at which solute comes out of solution (forming a solid precipitate).
The saturated solution is stable and said to be in dynamic equilibrium.

9. Solution Stability
In an unsaturated solution more solute can dissolve, so it is not yet at equilibrium.
In a supersaturated solution more than the maximum amount of solute that can normally be dissolved at the current conditions is present. This is an unstable situation that is resolved by precipitating solute (solid). The end result is an equilibrium condition.

10. Saturation:

11. Quantitative Descriptions of Solutions
Quantitative methods are much more useful than qualitative descriptions because they specify the amounts of components in solutions.
The most common quantitative descriptions include:
Molarity, M = moles solute/L of solution
Molality, m = moles of solute/kg of solvent

12. Molarity, M
Molarity (M) – moles of solute per liter of solution M = moles/L
What is the molarity of a solution made from 145 g of NaCl in 2.75 L of solution?
Vinegar is a solution of acetic acid. What is the molarity of the solution produced when 125g of acetic acid (C2H4O2) is dissolved in sufficient water to prepare 1.50L of solution?

13. Molality, m
Molality (m) – moles of solute per kilogram of solvent.
m = moles/kg
What is the molality of a solution made from 20.4 g KBr in 195 g of water?
What is the molality of a solution containing 125g of iodine (I2) and 750.g of carbon tetrachloride (CCl4)?

14. Mole Fraction, xsolute Xsolute = moles solute
Mole Fraction – moles of component per total moles of solution.
Xsolute = moles solute
total moles
Example: What is the mole fraction of sulfur dioxide in an industrial exhaust gas containing g of SO2 dissolved in every g of CO2?
Answer:
XSO2 = (mole fraction SO2)/(total moles of solution)
Moles SO2 = 128.0g • (1 mol SO2 /64.04g SO2) = mol SO2
Moles CO2 = 1500.g CO2 • [1 mol CO2/44.01g CO2] = mol CO2
XSO2 = mol SO2 ___________ = =
1.999 mol SO mol CO

15. Molarity and Dilution Factors
When diluting a solution into a less concentrated one, the total number of moles of solute does not change.
(The compound didn’t go anywhere; it is still in the container!)
Because Molarity = moles/volume,
the moles = Molarity * Volume, or M V.
Therefore we may write the following for the each of the two solutions:
moles1 = M1V1 and moles2 = M2V2
Since there are the same number of moles in the first solution as in the second, we may let moles1 = moles2, or also
McVc = MdVd
Can use this simple equation to calculate the new molarity.

16. More Solution Definitions to Know
Miscible – liquids that may be mixed together in any amount.
Oil and gasoline.
Immiscible – liquids that cannot be mixed.
Oil and water.
Aqueous Solution – liquid solutions for which the solvent is water.
Acetic acid and water (vinegar).
Electrolyte – a substance that forms ions in solution, enabling the solution to conduct electricity.
NaCl in sea water, Gatorade.
Non-electrolyte – a substance that does not form ions in solution, thus giving a non-conducting solution.
Sugar in tea.

18. 15-3 Formation of Solutions
Dissolution - the complex interaction of two or more separate substances (the solute and the solvent) to form a single system (the solution).
Solvation – the process whereby solvent particles pull the solute particles into solution and surround them; the interaction between solute and solvent particles to form a solution.
Hydration - the process whereby water particles pull the solute particles into solution and surround them to form a solution.
Solubility – a quantifiable measure of the degree to which a substance dissolves in another substance; it is the amount of a solute that will dissolve in a specific solvent under given conditions. Expressed in gram of solute per 100 grams of solvent.

19. Saturation:

20. Factors Affecting Solubility
Nature of Solute and Solvent
Similar substances dissolve in one another. (“Likes dissolve likes.”)
Polar substances dissolve in polar substances
Water dissolves sugar & salt.
Water dissolves rubbing alcohol.
Nonpolar substances dissolve in nonpolar substances.
Gasoline dissolves oil.
Dry cleaning fluids dissolve grease and oils.
Temperature – see next slides
Pressure
Gas solubility increases with pressure.

21. Solubility and Temperature

22. Solubility and Temperature
Solubility of solids and liquids generally increases with temperature.
Solubility of gases decreases with temperature.
Solubility of salts:
Solubility of gases:

23. Energy Changes and the Formation of Solutions
Solvation/Hydration may be
exothermic or endothermic.
ΔH depends on the balance of energy released by the attraction of solute particles to the solvent versus the energy consumed in breaking the attractions of solute particles for each other (crystal lattice energy).
Dissolving CaCl2 is very exothermic, but an ammonium nitrate ‘cold pack’ works because the solvation is endothermic.

24. Factors That Affect Dissolution Rates
Surface area
Increasing surface area (making smaller particles) increases the rate of dissolution.
Stirring
Stirring the solution increases the rate of dissolution.
Temperature
Increasing the temperature increases the rate of dissolution.

25. 15-4 Colligative Properties
These are properties that depend on solution concentration rather than the nature or type of solute.
They are dependent on molality (molsolute/kgsolvent)
Examples include…
Vapor Pressure Reduction
Boiling Point Elevation
Freezing Point Depression

26. Vapor Pressure Reduction
Raoult’s Law – the magnitude of the vapor pressure reduction is proportional to the solute concentration, regardless of the solute.
Why? Nonvolatile solute molecules interfere with the solvent molecules, preventing them from leaving the surface of the solution, and thus decreasing the vapor pressure.
(Fig , p 520)
This results in an increase in the boiling point
of the solvent, and a decrease in its freezing point.
Boiling point elevation
Freezing point depression
Applications?

27. “Freezing Point Depression”
This is the ability of a dissolved solute to lower the freezing point of a solution.
Example: Antifreeze is added to a car’s coolant system to prevent freezing of the water in winter.
Decrease of freezing point is directly proportional to the molality (m) of the solute.
Calculated from ΔTf = Kfm
…where ΔTf is the temperature depression, m is molality and Kf is the freezing point depression constant.

28. “Boiling Point Elevation”
This is the ability of a dissolved solute to raise the boiling point of a solution.
Example: The antifreeze added to a car’s coolant system also prevents overheating in summer!
Increase of boiling point is also directly proportional to the molality (m) of the solute.
Calculated from ΔTb = Kbm
…where ΔTb is the temperature depression, m is molality and Kb is the freezing point depression constant.

29. Post Lab Questions
1. Which compound types (ionic or covalent) produce more particles when dissolved in water and why? Remember this is related to molality.
 2. Which type of compound (ionic or covalent) will have a greater effect on the colligative properties of a solution? Explain.   

30. Determining Molar Mass
A solution containing 16.9g of a nonvolatile molecular compound in 250g of water has a freezing point of °C. What is the molar mass of the compound?
Solution:
First find the molality from ΔTf = Kfm.
0.744 °C = (1.86 °C/m) x m,
so the molality = molal
But molality = moles/kg, so
0.400 m = ??? moles/0.250 kg,
from which ??? moles = mole
-Since mole = 16.9g of solute, 1.00 mole of solute = 169 g, giving a molar mass of 169 g/mol.