Lecture 38 (Slides) November 14

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Molarity and Molality
Amount of solute (in moles)
Molarity (M) =
Volume of solution (in liters)
Amount of solute (in moles)
Molality (m) =
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Mass of solvent (in kilograms)
General Chemistry: Chapter 13
Slide 1 of 46
Molarity and Molality
• For dilute aqueous solutions the molality and
molality of a solution are usually very similar.
• Why is this the case?
Class Examples
• 2. A solution is prepared by dissolving 44.6g of
Cu(NO3)2.6H2O(s) in enough water to make
825 mL of solution. What is the molar
concentration of Cu2+(aq) ions and NO3-(aq)
ions in this solution?
• 3. 2.25 L of 0.400 mol.L-1 Al(NO3)3 (aq) and
2.00L of 0.350 mol.L-1 Ba(NO3)2 (aq) are
mixed. What is the molar concentration of
nitrate ions in the resulting solution?
Physical Properties – Concentrations: :
• The most useful concentration units for
physical properties studies show the relative
numbers of molecules (or ions) of each
substance. The relative number of molecules
(of each substance) is the same as the relative
number of moles (of each substance). Often we
employ mole fractions – especially for vapor
pressure calculations.
Mole Fraction and Mole Percent
i =
Amount of component i (in moles)
Total amount of all components (in moles)
1 + 2 + 3 + …n = 1
Mole % i = i  100%
Copyright © 2011 Pearson
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General Chemistry: Chapter 13
Slide 5 of 46
Molarity and Molality
• Molarity (mol∙L-1), does not indicate the
relative amounts of solute(s) and solvent. The
next slide helps demonstrate why. An alternate
concentration unit, molality, does give an
indication of the relative amounts of solute(s)
and solvent. We can convert from molarity to
molality given the solution density.
Molarity and Molality
Amount of solute (in moles)
Molarity (M) =
Volume of solution (in liters)
Amount of solute (in moles)
Molality (m) =
Copyright © 2011 Pearson
Canada Inc.
Mass of solvent (in kilograms)
General Chemistry: Chapter 13
Slide 7 of 46
Class Example
• 4. At 25 o C a concentrated H2SO4/water
solution has a density of 1.841 g.cm-3 and is
95.1 % H2SO4 by mass.
• Find: (a) the molarity of H2SO4.
• (b) the molarity of H2SO4
• (c) the mole fraction of H2SO4 and water in the
solution.
Intermolecular Forces and the Solution
Process
FIGURE 13-2
•Enthalpy diagram for solution formation
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General Chemistry: Chapter 13
Slide 9 of 46
Intermolecular Forces in Mixtures
Magnitude of ΔHa, ΔHb, and ΔHc
depend on intermolecular forces.
Ideal solution
Forces are similar between all
combinations of components.
ΔHsoln = 0
FIGURE 13-3
•Intermolecular forces in a solution
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General Chemistry: Chapter 13
Slide 10 of 46
Similar Intermolecular Forces
• Molecules with similar structures often have
intermolecular forces of the same type and of
similar strength. The next slide shows the
structures of benzene and the slightly more
complex toluene molecule. What
intermolecular forces are important for these
two molecules?
FIGURE 13-4
Two components of a nearly ideal solution
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General Chemistry: Chapter 13
Slide 12 of 46
Formation of Ionic Solutions
FIGURE 13-6
•An ionic crystal dissolving in water
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General Chemistry: Chapter 13
Slide 13 of 46
Hydrated Ions – Intermolecular Forces
• Can highly polar water molecules and ions
interact? Yes. This is represented on the
previous slide (2 dimensions!). The interaction
is particularly important for small metal ions
with larger charges – such as Mg2+(aq).
(Aside: The effects of hydration are sometimes
surprising – thus, e.g., lithium ions move
through water more slowly than potassium
ions!)
Solution Formation and Equilibrium
FIGURE 13-7
•Formation of a saturated solution
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General Chemistry: Chapter 13
Slide 15 of 46
13-5 Solubility of Gases
Effect of Temperature
•Most gases are less
soluble in water as
temperature increases.
•In organic solvents the
reverse is often true.
Effect of temperature on
the solubilities of gases
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General Chemistry: Chapter 13
Slide 16 of 46
Dissolved Oxygen
• The oxygen that dissolves in fresh and sea
water is critical to aquatic life/food chains. The
amount of dissolved oxygen decreases as
water temperature increases which is an
important current concern. In aquatic
environments oxygen levels can drop due to
agricultural runoff as well (Gulf of Mexico
“dead zones”).
Henry’s Law ? – Global Warming?
Effect of Pressure
•William Henry found that the solubility of a gas increases with
increasing pressure.
C = kPgas
k=
Pgas =
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C
Pgas
C
k
23.54 mL
1.00 atm
=
=
= 23.54 ml N2/atm
100 mL
= 4.25 atm
23.54 ml N2/atm
General Chemistry: Chapter 13
Slide 19 of 46
FIGURE 13-11
Effect of pressure on the solubility of a gas
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General Chemistry: Chapter 13
Slide 20 of 46
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General Chemistry: Chapter 13
Slide 21 of 46
Henry’s Law Example
• 5. What are the expected units for the Henry’s
Law constant, kH. What graph might you draw
(at least mentally) to remind yourself of the
form of the Henry’s Law equation and, as well,
the units for kH?
• 6. The concentration of a dissolved gas with at
a “surface pressure” P1 is c1. What are two
ways in which we could calculate the
concentration of dissolved gas , c2, at a
pressure P2?
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