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)

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(NO 3 ) 2. 6H 2 O(s) in enough water to make 825 mL of solution. What is the molar concentration of Cu 2+ (aq) ions and NO 3 - (aq) ions in this solution? L of mol. L -1 Al(NO 3 ) 3 (aq) and 2.00L of mol. L -1 Ba(NO 3 ) 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 Copyright © 2011 Pearson Canada Inc. Slide 5 of 46 General 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%

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 Copyright © 2011 Pearson Canada Inc. Slide 7 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)

Class Example 4. At 25 o C a concentrated H 2 SO 4 /water solution has a density of g. cm -3 and is 95.1 % H 2 SO 4 by mass. Find: (a) the molarity of H 2 SO 4. (b) the molarity of H 2 SO 4 (c) the mole fraction of H 2 SO 4 and water in the solution.

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

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 ΔH soln = 0 Magnitude of ΔH a, ΔH b, and ΔH c depend on intermolecular forces. Ideal solution Forces are similar between all combinations of components.

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?

Two components of a nearly ideal solution FIGURE 13-4 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 13 Slide 12 of 46

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

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 Mg 2+ (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 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 13 Slide 15 of 46 FIGURE 13-7 Formation of a saturated solution

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.

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 Copyright © 2011 Pearson Canada Inc. Slide 19 of 46 General Chemistry: Chapter 13 William Henry found that the solubility of a gas increases with increasing pressure. C = kP gas k = C P gas = mL 1.00 atm = ml N 2 /atm k C P gas = = 100 mL = 4.25 atm ml N 2 /atm

Effect of pressure on the solubility of a gas FIGURE Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 13 Slide 20 of 46

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

Henry’s Law Example 5. What are the expected units for the Henry’s Law constant, k H. 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 k H ? 6. The concentration of a dissolved gas with at a “surface pressure” P 1 is c 1. What are two ways in which we could calculate the concentration of dissolved gas, c 2, at a pressure P 2 ?

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