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Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume of solution (in liters) Molality (m) = Amount of solute (in moles) Mass of solvent (in kilograms)

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Page 1: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Molarity and Molality

Copyright © 2011 Pearson Canada Inc. Slide 1 of 46General Chemistry: Chapter 13

Molarity (M) = Amount of solute (in moles)

Volume of solution (in liters)

Molality (m) = Amount of solute (in moles)

Mass of solvent (in kilograms)

Page 2: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Molarity and Molality

• For dilute aqueous solutions the molality and molality of a solution are usually very similar.

• Why is this the case?

Page 3: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

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?

Page 4: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

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.

Page 5: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Mole Fraction and Mole Percent

Copyright © 2011 Pearson Canada Inc. Slide 5 of 46General Chemistry: Chapter 13

i = Amount of component i (in moles)

Total amount of all components (in moles)

1 + 2 + 3 + …n = 1

Mole % i = i 100%

Page 6: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

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.

Page 7: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Molarity and Molality

Copyright © 2011 Pearson Canada Inc. Slide 7 of 46General Chemistry: Chapter 13

Molarity (M) = Amount of solute (in moles)

Volume of solution (in liters)

Molality (m) = Amount of solute (in moles)

Mass of solvent (in kilograms)

Page 8: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

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.

Page 9: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Intermolecular Forces and the Solution Process

Copyright © 2011 Pearson Canada Inc.

General Chemistry: Chapter 13 Slide 9 of 46

FIGURE 13-2

•Enthalpy diagram for solution formation

Page 10: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Intermolecular Forces in Mixtures

Copyright © 2011 Pearson Canada Inc.

General Chemistry: Chapter 13 Slide 10 of 46

FIGURE 13-3•Intermolecular forces in a solution

ΔHsoln = 0

Magnitude of ΔHa, ΔHb, and ΔHc depend on intermolecular forces.

Ideal solution

Forces are similar between all combinations of components.

Page 11: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

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?

Page 12: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Two components of a nearly ideal solutionFIGURE 13-4

Copyright © 2011 Pearson Canada Inc.

General Chemistry: Chapter 13 Slide 12 of 46

Page 13: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Formation of Ionic Solutions

Copyright © 2011 Pearson Canada Inc.

General Chemistry: Chapter 13 Slide 13 of 46

FIGURE 13-6

•An ionic crystal dissolving in water

Page 14: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

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!)

Page 15: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Solution Formation and Equilibrium

Copyright © 2011 Pearson Canada Inc.

General Chemistry: Chapter 13 Slide 15 of 46

FIGURE 13-7

•Formation of a saturated solution

Page 16: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

13-5 Solubility of Gases

Copyright © 2011 Pearson Canada Inc.

General Chemistry: Chapter 13 Slide 16 of 46

Effect of temperature on the solubilities of gases

Effect of Temperature

•Most gases are less soluble in water as temperature increases.

•In organic solvents the reverse is often true.

Page 17: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

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”).

Page 18: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Henry’s Law ? – Global Warming?

Page 19: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Effect of Pressure

Copyright © 2011 Pearson Canada Inc. Slide 19 of 46General Chemistry: Chapter 13

•William Henry found that the solubility of a gas increases with increasing pressure.

C = kPgas

k = C

Pgas

=23.54 mL

1.00 atm= 23.54 ml N2/atm

k

CPgas = =

100 mL= 4.25 atm

23.54 ml N2/atm

Page 20: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Effect of pressure on the solubility of a gasFIGURE 13-11

Copyright © 2011 Pearson Canada Inc.

General Chemistry: Chapter 13 Slide 20 of 46

Page 21: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Copyright © 2011 Pearson Canada Inc. Slide 21 of 46General Chemistry: Chapter 13

Page 22: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

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?

Page 23: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume

Copyright 2011 Pearson Canada Inc. 13 - 23

Page 24: Molarity and Molality Copyright © 2011 Pearson Canada Inc. Slide 1 of 46 General Chemistry: Chapter 13 Molarity (M) = Amount of solute (in moles) Volume