MEC Chapter 6

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Chapter 6
Solutions
Denniston
Topping
Caret
7th Edition

2. 6.1 Properties of Solutions
• Solution - homogeneous mixture
• Solute - the substance in the mixture present
in lesser quantity
• Solvent - the substance present in the largest
quantity
• Aqueous solution - solution where the
solvent is water
• Solutions can be liquids as well as solids and
gases

3. 6.1 Properties of Solutions Examples of Solutions
• Air - oxygen and several trace gases are
dissolved in the gaseous solvent, nitrogen
• Alloys - brass and other homogeneous
metal mixtures in the solid state
• Focus on liquid solutions as many important
chemical reactions take place in liquid
solutions

4. 6.1 Properties of Solutions General Properties of Liquid
Solutions
• Clear, transparent, no visible particles
• May have color
• Electrolytes are formed from solutes that are
soluble ionic compounds
• Nonelectrolytes do not dissociate
NaCl(s ) H→ Na + (aq ) + Cl- (aq )
2O
• Volumes of solute and solvent are not additive
– 1 L ethanol + 1 L water does not give 2 L of solution

5. 6.1 Properties of Solutions
Solutions and Colloids
• Colloidal suspension - contains solute
particles which are not uniformly
distributed
– Due to larger size of particles (1nm - 200 nm)
– Appears identical to solution from the
naked eye
– Smaller than 1 nm, have solution
– Larger than 1 nm, have a precipitate

6. 6.1 Properties of Solutions Degree of Solubility
• Solubility - how much of a particular solute can
dissolve in a certain solvent at a specified
temperature
• Factors which affect solubility:
1 Polarity of solute and solvent
• The more different they are, the lower the solubility
2 Temperature
• Increase in temperature usually increases solubility
3 Pressure
• Usually has no effect
• If solubility is of gas in liquid, directly proportional
to applied pressure

7. 6.1 Properties of Solutions Saturation
• Saturated solution - a solution that contains all the
solute that can be dissolved at a particular
temperature
• Supersaturated solution - contains more solute
than can be dissolved at the current temperature
• How is this done?
• Heat solvent, saturate it with solute then cool slowly
• Sometimes the excess will precipitate out
• If it doesn’t precipitate, the solution will be
supersaturated

8. 6.1 Properties of Solutions Solubility and Equilibrium
• If excess solute is added to a solvent, some
dissolves
• At first, rate of dissolution is large
• Later, reverse reaction – precipitation – occurs
more quickly
• When equilibrium is reached the rates of
dissolution and precipitation are equal, there is
some dissolved and some undissolved solute
• A saturated solution is an example of a dynamic
equilibrium

9. 6.1 Properties of Solutions Solubility of Gases: Henry’s Law
• Henry’s law – the number of moles of a gas
dissolved in a liquid at a given temperature is
proportional to the partial pressure of the gas
above the liquid
• Gas solubility in a liquid is directly proportional to
the pressure of the gas in the atmosphere in
contact with the liquid
• Gases are most soluble at low temperatures
• Solubility decreases significantly at higher
temperatures
– Carbonated beverages – CO2 solubility less when warm
– Respiration – facilitates O2 and CO2 exchange in lungs

10. 6.2 Concentration Based on Mass
6
• Concentration - amount of solute dissolved
in a given amount of solution
• Concentration of a solution has an effect on
– Physical properties
• Melting and boiling points
– Chemical properties
• Solution reactivity

11. 6.2 Concentration Based on
Weight/Volume Percent
• Amount of solute = mass of solute in grams
• Amount of solution = volume in milliliters
amount of solute
concentration =
Mass
amount of solution
• Express concentration as a percentage by
multiplying ratio by 100% = weight/volume
percent or % (W/V)
W grams of solute
% = ×100%
V milliliters of solution

12. 6.2 Concentration Based on Calculating Weight/Volume
Percent
Calculate the percent composition or % (W/V) of
2.00 x 102 mL containing 20.0 g sodium chloride
20.0 g NaCl, mass of solute
Mass
2.00 x 102 mL, total volume of solution
% (W/V) = 20.0g NaCl / 2.00 x 102 mL x 100%
= 10.0% (W/V) sodium chloride

13. Calculate Weight of Solute from
6.2 Concentration Based on
Weight/Volume Percent
Calculate the number of grams of glucose in
7.50 x 102 mL of a 15.0% solution
Mass
W grams of solute
% = × 100%
V milliliters of solution
15.0% (W/V) = Xg glucose/7.50 x 102 mL x 100%
Xg glucose x 100% = (15.0% W/V)(7.50 x 102 mL)
Xg glucose = 113 g glucose

14. 6.2 Concentration Based on Weight/Weight Percent
W grams solute
% = ×100%
W grams solutions
• Weight/weight percent is most useful for
Mass
solutions of 2 solids whose masses are
easily obtained
• Calculate % (W/W) of platinum in gold
ring with 14.00 g Au and 4.500 g Pt
[4.500 g Pt / (4.500 g Pt + 14.00 g Au)] x 100%
= 4.500 g / 18.50 g x 100% = 24.32% Pt

15. 6.3 Concentration of Solutions:
Moles and Equivalents
• Chemical equations represent the relative
number of moles of reactants producing
products
• Many chemical reactions occur in solution
where it is most useful to represent
concentrations on a molar basis

16. 6.3 Moles and Equivalents Molarity
• The most common mole-based
concentration unit is molarity
• Molarity
– Symbolized M
– Defined as the number of moles of solute per
liter of solution
moles solute
M=
L solution

17. 6.3 Moles and Equivalents Calculating Molarity from Moles
• Calculate the molarity of 2.0 L of
solution containing 5.0 mol NaOH
• Use the equation moles solute
M=
L solution
• Substitute into the equation:
MNaOH = 5.0 mol solute
2.0 L solution
= 2.5 M

18. 6.3 Moles and Equivalents Calculating Molarity From Mass
• If 5.00 g glucose are dissolved in 1.00 x 102 mL of
solution, calculate molarity, M, of the glucose solution
• Convert from g glucose to moles glucose
– Molar mass of glucose = 1.80 x 102 g/mol
5.00 g x 1 mol / 1.80 x 102 g = 2.78 x 10-2 mol glucose
– Convert volume from mL to L
1.00 x 102 mL x 1 L / 103 mL = 1.00 x 10-1 L
• Substitute into the equation:
moles solute
M=
L solution
Mglucose = 2.78 x 10-2 mol glucose
1.00 x 10-1 L solution
= 2.78 x 10-1 M

19. 6.3 Moles and Equivalents Dilution
Dilution is required to prepare a less
concentrated solution from a more
concentrated one
– M1 = molarity of solution before dilution
– M2 = molarity of solution after dilution
– V1 = volume of solution before dilution
– V2 = volume of solution after dilution
moles solute
M= moles solute = (M)(L solution)
L solution

20. 6.3 Moles and Equivalents Dilution
• In a dilution will the
number of moles of solute
change?
– No, only fewer per unit
volume
• So, M1V1 = M2V2
• Knowing any three terms
permits calculation of the
fourth

21. 6.3 Moles and Equivalents Calculating Molarity
After Dilution
• Calculate the molarity of a solution made by
diluting 0.050 L of 0.10 M HCl solution to a
volume of 1.0 L
– M1 = 0.10 M molarity of solution before dilution
– M2 = X M molarity of solution after dilution
– V1 = 0.050 L volume of solution before dilution
– V2 = 1.0 L volume of solution after dilution
• Use dilution expression M1V1 = M2V2
• X M = (0.10 M) (0.050 L) / (1.0 L)
0.0050 M HCl OR 5.0 x 10-3 M HCl

22. 6.3 Moles and Equivalents Representation of Concentration
of Ions in Solution
Two common ways of expressing
concentration of ions in solution:
1. Moles per liter (molarity)
• Molarity emphasizes the number of
individual ions
2. Equivalents per liter (eq/L)
• Emphasis on charge

23. 6.3 Moles and Equivalents Comparison of Molarity and
Equivalents
1 M Na3PO4
• What would the concentration of PO43- ions be?
• 1M
• Equivalent is defined by the charge
• One Equivalent of an ion is the number of grams
of the ion corresponding to Avogadro’s number of
electrical charges
molar mass of ion (g)
One equivalent of an ion =
number of charges on ion

24. 6.3 Moles and Equivalents Molarity vs. Equivalents – 1 M Na3PO4
• 1 mol Na+ = 1 equivalent Na+
• 1 mol PO43- = 3 equivalents PO43-
• Equivalents of Na+?
– 3 mol Na+ = 3 equivalents of Na+
• Equivalents of PO43-?
– 1 mol PO43- = 3 equivalents of PO43-

25. 6.3 Moles and Equivalents Calculating Ion Concentration
• Calculate eq/L of phosphate ion, PO43- in a
solution with 5.0 x 10-3 M phosphate
• Need to use two conversion factors:
– mol PO43- mol charge
– mol charge eq PO43
5.0 x 10-3 mol PO43- x 3 mol charge x 1 eq
1L 1 mol PO43- 1mol charge
• 1.5 x 10-2 eq PO43- /L

26. 6.4 Concentration-Dependent
Solution Properties
• Colligative properties - properties of
solutions that depend on the concentration
of the solute particles, rather than the
identity of the solute
• Four colligative properties of solutions
1. vapor pressure lowering
2. boiling point elevation
3. freezing point depression
4. osmotic pressure

27. 6.4 Concentration-Dependent Vapor Pressure of a Liquid
Consider Raoult’s law in molecular
Solution Properties
terms
• Vapor pressure of a solution
results from escape of solvent
molecules from liquid to gas
phase
• Partial pressure of gas phase
solvent molecules increases
until equilibrium vapor
pressure is reached
• Presence of solute molecules
hinders escape of solvent
molecules, lowering
equilibrium vapor pressure

28. 6.4 Concentration-Dependent Vapor Pressure Lowering
• Raoult’s law - when a nonvolatile solute is
Solution Properties
added to a solvent, vapor pressure of the solvent
decreases in proportion to the concentration of
the solute
• Solute molecules (red below) serve as a barrier to
the escape of solvent molecules resulting in a
decrease in the vapor pressure

29. 6.4 Concentration-Dependent Freezing Point Depression and
Solution Properties Boiling Point Elevation
• Freezing point depression may be explained
considering the equilibrium between solid and
liquid states
– Solute molecules interfere with the rate at which
liquid water molecules associate to form the solid
state
• Boiling point elevation can be explained
considering the definition as the temperature at
which vapor pressure of the liquid equals the
atmospheric pressure
– If a solute is present, then the increase in boiling
temperature is necessary to raise the vapor pressure
to atmospheric temperature

30. 6.4 Concentration-Dependent Freezing Point Depression
• Freezing point depression (∆Tf) - is proportional
Solution Properties
to the number of solute particles
– Solute particles, not just solute
• How does an electrolyte behave?
– Dissociate into ions
• An equal concentration of NaCl will affect the
freezing point twice as much as glucose (a
nonelectrolyte)
• Each solvent has a unique freezing point
depression constant or proportionality factor
∆Tf=kf m

31. 6.4 Concentration-Dependent Boiling point elevation
• Boiling point elevation (∆Tb) - is
Solution Properties
proportional to the number of solute
particles
• An electrolyte will affect boiling point to
a greater degree than a nonelectrolyte of
the same concentration
• Each solvent has a unique boiling point
elevation constant
∆Tb=kb m

32. 6.4 Concentration-Dependent Osmotic Pressure
• Some types of membranes appear impervious
Solution Properties
to matter, but actually have a network of small
holes called pores
• These pores may be large enough to permit
small solvent molecules to move from one side
of the membrane to the other
• Solute molecules cannot cross the membrane as
they are too large
• Semipermeable membrane - allows
solvent but not solute to diffuse from one side
to another

33. 6.4 Concentration-Dependent
Osmotic Pressure
• Osmosis - the
Solution Properties
movement of
solvent from a
dilute solution to a
more concentrated
solution through a
semipermeable
membrane
• Requires pressure
to stop this flow

34. 6.4 Concentration-Dependent Osmotic Pressure
Solution Properties
• Osmotic pressure (π) - the amount of
pressure required to stop the flow across
a semipermeable membrane
π=MRT
• Osmolarity - the molarity of particles in
solution
– Osmol, used for osmotic pressure
calculation

35. 6.4 Concentration-Dependent Tonicity and the Cell
• Living cells contain aqueous solution and these cells
Solution Properties
are also surrounded by aqueous solution
• Cell function requires maintenance of the same osmotic
pressure inside and outside the cell
• Solute concentration of fluid surrounding cells higher
than inside results in a hypertonic solution causing
water to flow into the surroundings, causing collapse =
crenation
• Solute concentration of fluid surrounding cells too low,
results in a hypotonic solution causing water to flow
into the cell, causing rupture = hemolysis
• Isotonic solutions have identical osmotic pressures and
no osmotic pressure difference across the cell
membrane

36. 6.4 Concentration-Dependent Tonicity and the Cell
Solution Properties
Crenation Hemolysis Isotonic

37. 6.4 Concentration-Dependent Pickling Cucumber in Hypertonic
Solution Properties Brine Due to Osmosis

38. 6.5 Water as a Solvent
• Water is often referred to as the “universal
solvent”
• Excellent solvent for polar molecules
• Most abundant liquid on earth
• 60% of the human body is water
– transports ions, nutrients, and waste into and out of
cells
– solvent for biochemical reactions in cells and
digestive tract
– reactant or product in some biochemical processes