1. Properties of Solutions

2. Learning objectives Define terms solute, solvent and solution
Distinguish between solutions and heterogeneous mixtures
Distinguish among non-, weak and strong electrolytes
Describe factors that affect solubility
Describe Henry’s law and its application to explain common phenomena involving gases
Perform calculations of solution concentration using various definitions
Use molarity in stoichiometry calculations
Describe basis of Raoult’s law and colligative properties
Calculate solute concentrations in colligative property context
Explain basis of osmotic pressure

3. Definitions of a solution
A homogeneous mixture of two or more substances
Solute is the component that is dispersed in the solvent – usually the minority component
Solvent is the dispersing component – usually the majority component
Sometimes definitions can become blurred: water (solvent) dissolves much greater than its own mass of sugar (solute)

5. Formation of a solution
Crystals are held together by strong ionic bonds
Polar water molecules exert attractive forces on ions
Hydration of the ions by water molecules overcomes lattice energy
Crystal lattice disperses

6. Like dissolves like
All gases mix with each because there are no intermolecular forces
Solids and liquids mix if intermolecular forces between unlike substances (adhesive forces) are similar to forces between like substances (cohesive forces)

7. Electrolytes
Electrolytes are substances that dissociate into ions in solution – ionic compounds (sodium chloride)
Strong electrolytes are completely ionized
Weak electrolytes are partly ionized
Non-electrolytes are those substances that produce no ions (sugar)

9. Colloids and solutions
Both appear clear and uniform
Solution is homogeneous; colloid is heterogeneous
Colloid contains particles suspended in the liquid
1 – 200 nm diameter
Colloid particles scatter light, solute particles do not

10. Factors affecting solubility
Difference in polarity between solute and solvent – like dissolves like
Temperature
Solid solutes: depends on balance of several factors – can increase, decrease or stay the same
Gases: solubility always decreases with temperature
Pressure
Solids: little influence
Gases: solubility always increases with pressure (Henry’s law)

11. Saturation
A saturated solution is one which is in equilibrium with undissolved solute – it has reached limit of solubility
Supersaturation arises when amount of substance in solution is greater than that predicted on basis of saturation. An essential condition for the growth of crystals

12. Henry’s Law
The number of moles of gas dissolved in a liquid is proportional to the partial pressure of the gas
Exchange of CO2 and O2 in respiration depends on Henry’s Law.
In the lungs, the O2 partial pressure is higher than that of CO2
In the blood, the CO2 pressure is higher after respiration

13. Real world applications 1: Henry’s Law and sodie pop
The quantity of gas dissolved in a liquid depends directly on the pressure of that gas above the liquid
Under pressure the CO2 in the liquid is kept in solution
Open the cap and the CO2 rapidly escapes

14. Real world applications 2: The science of breathing
The gas laws explain the mechanics of breathing: the transport of oxygen from the lungs and exchange with carbon dioxide produced in the body.

15. Measuring concentration
Concentration = amount of solute/amount of solution
Weight/volume percent
Mass solute in g/volume of soln in mL x 100%
Weight/weight percent
Mass solute in g/mass solution in g x 100%

16. Molarity Concentration is usually expressed in terms of molarity:
Moles of solute/liters of solution (M)
Moles of solute = molarity x volume of solution
Moles = M x V

17. Molarity and concentration
M = moles solute/liter of solution
Dilution
M1V1 = M2V2
Dilution factor = V2/V1 (V2>V1)
M2<M1

18. Example What is molarity of 50 ml solution containing 2.355 g H2SO4?
Molar mass H2SO4 = 98.1 g/mol
Moles H2SO4 = mol (2.355 g/98.1 g/mol)
Volume of solution = 50 mL/1000 mL/L = .050 L
Concentration = moles/volume
= mol/.050 L = M

19. Solution stoichiometry
How much volume of one solution to react with another solution
Given volume of A with molarity MA
Determine moles A
Determine moles B
Find target volume of B with molarity MB

20. Titration
Use a solution of known concentration to determine concentration of an unknown
Must be able to identify endpoint of titration to know stoichiometry
Most common applications with acids and bases

21. Colligative properties
Properties that depend upon the concentration of solute particles but not their identity
Vapor pressure lowering
Freezing point depression
Boiling point elevation
Osmotic pressure

22. Raoult’s law
When nonvolatile solute is added to solvent, vapor pressure of solvent decreases in proportion to concentration of solute
Freezing point goes down
Boiling point goes up

23. Freezing and melting are dynamic processes
At equilibrium, rate of freezing = rate of melting

24. Adding salts upsets the equilibrium
Fewer water molecules at surface: rate of freezing drops
Ice turns into liquid
Lower temperature to regain balance
Depression of freezing point

25. The same model explains elevated boiling point
Condensation and evaporation are dynamic processes
Replacing some of the liquid water with salt reduces rate of evaporation – leads to condensation
Raise temperature to recover balance

26. Mathematical base Freezing point depression Boiling point elevation
ΔTf = kf x solute concentration
Boiling point elevation
ΔTb = kb x solute concentration

27. Units of concentration
Effect depends upon number of particles not mass of particles, so concentration must be in moles.
Molality (m) is used in these situations
Moles solute/kg solvent
Temperature independent measure of concentration

28. Type of solute important
Covalent solute produces one particle per molecule:
C6H12O6 (s) → C6H12O6 (aq)
Ionic solutes produce >1 particle per formula unit:
NaCl (s) → Na+(aq) + Cl-(aq) (2 particles)
CaCl2(s) → Ca2+(aq) + 2Cl-(aq) (3 particles)

29. Osmotic pressure Transport across semipermeable membranes:
Solvent particles admitted but solute particles rejected
Osmosis involves passage of water molecules across a membrane

30. Osmosis
Transport of water molecules from dilute solution to more concentrated one
Imbalance of concentration provides driving force
Osmotic pressure is the pressure required to oppose this flow

31. Osmotic pressure Osmotic pressure is written as: πV = nRT
Temperature
Volume
No moles
Gas constant

32. Calculating osmotic pressure
But n/V = concentration in moles per liter =M
π = MRT (T in Kelvin)
But what molarity? Need to know moles of particles
C6H12O6 = 1 mole particles
NaCl = 2 moles of particles
CaCl2 = 3 moles of particles
Osmolarity refers to concentration of particles for osmotic pressure determination

33. Osmotic pressure and cells
Concentration in cells depends on osmosis
Concentration outside cell > inside (hypertonic) – crenation
Concentration outside cell < inside (hypotonic) - hemolysis
Crenation
Hemolysis