1 CHAPTER 4 Solutions B By Dr. Hisham Ezzat First year

2 Example 6 Calculate the molarity and the molality of an aqueous solution that is 10.0% glucose, C 6 H 12 O 6. The density of the solution is 1.04 g/mL. 10.0% glucose solution has several medical uses. 1 mol C 6 H 12 O 6 = 180 g

3 Calculate the molality and the molarity of an aqueous solution that is 10.0% glucose, C 6 H 12 O 6. The density of the solution is 1.04 g/mL. 10.0% glucose solution has several medical uses. 1 mol C 6 H 12 O 6 = 180 g You calculate the molarity!

4 Example 7 Calculate the molality of a solution that contains 7.25 g of benzoic acid C 6 H 5 COOH, in 2.00 x 10 2 mL of benzene, C 6 H 6. The density of benzene is g/mL. 1 mol C 6 H 5 COOH = 122 g You do it!

5 Example 8 What are the mole fractions of glucose and water in a 10.0% glucose solution (Example 6)? You do it!

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7 Now we can calculate the mole fractions.

Chapter 13 8 The extent to which a solute dissolves in solvent depends The nature of the solute. The nature of the solvent. The temperature. The pressure (for gases).

Few organic compounds that dissolve readily in water, most contain - OH groups. methyl alcohol, ethyl alcohol, and ethylene glycol, all of which are soluble in water in all proportions. 9

(Miscibility Pairs of liquids that mix in any proportions are said to be miscible. Example: Ethanol and water are miscible liquids. In contrast, immiscible liquids do not mix significantly. Example: Gasoline and water are immiscible. 10

(Miscibility For example, methanol, CH 3 OH, is very soluble in water 11

(Miscibility Water and ethanol are miscible because the broken hydrogen bonds the more C atoms in the alcohol, the lower its solubility in water. Increasing the number of –OH groups within a molecule increases its solubility in water. 12

13 (Miscibility) Nonpolar molecules essentially “slide” in between each other. تنزلق  For example, carbon tetrachloride and benzene are very miscible.

Experience tells us that sugar dissolves better in warm water than in cold water. As temperature increases, solubility of solids generally increases. Sometimes solubility decreases as temperature increases (e.g., Ce 2 (SO 4 ) 3 ). Gases are less soluble at higher temperatures. An environmental application of this is thermal pollution. 14 Effect of Temperature on Solubility

15 Solvent Solution  H solvent  H solute solution Solute

16 exothermic Solute + solvent → solution + heat or endothermic Solute + solvent + heat → solution Δ H = H solution - (H solute + H solvent ). Δ H solution negative = exothermic positive. = endothermic,

17 Effect of Temperature on Solubility According to LeChatelier’s Principle when stress is applied to a system at equilibrium, the system responds in a way that best relieves the stress.  Since saturated solutions are at equilibrium, LeChatelier’s principle applies to them. Possible stresses to chemical systems include: 1. Heating or cooling the system. 2. Changing the pressure of the system. 3. Changing the concentrations of reactants or products.

Example of endothermic dissolution 21 kJ + KI(s)  K + + I - 18 equilibrium will shift to the right  using up some of the added heat (and some of the excess solid KI) and  increasing the concentration of K + and I - ions in solution. Le Chatelier's principle the solubility of KI increases with increasing temperature.

19 Temperature Effects

20 Example of exothermic dissolution solubility of lithium iodide decreases with an increase in temperature LiI (s)  Li + + I KJ equilibrium shifts to the left (1) using up some of the added heat (and Li and I - ions in solution) and, (2) forming more solid Lil. (We observe the precipitation of some Lil out of solution.)

21 Factors Affecting Solubility Temperature Effects the solubility of gas decreases with temperature.

Effect of Pressure on Solubility The effect of pressure on the solubility of gases in liquids is described by Henry’s Law. Pressure changes have little or no effect on solubility of liquids and solids in liquids. Liquids and solids are not compressible. Pressure changes have large effects on the solubility of gases in liquids. 22

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24 Henry’s Law – The solubility of a gas increases in direct proportion to its partial pressure above the solution.

25 Factors Affecting Solubility Henry’s Law – The solubility of a gas increases in direct proportion to its partial pressure above the solution. C g - solubility of gas P g - the partial pressure of the gas k - Henry’s law constant. Pressure Effects

26 Carbonated beverages are bottled under P CO 2 > 1 atm. As the bottle is opened, P CO 2 decreases and the solubility of CO 2 decreases. Therefore, bubbles of CO 2 escape from solution.

27 At 740 torr and 20°C, nitrogen has solubility in H 2 O of g /I. At 620 torr and 20°C its solubility is g/l. Do these data show that nitrogen obey Henry's law or not? Problem:

28 At 25°C oxygen gas collected over water at a total pressure of 101 kPa is soluble to the extent of g dm -3. What would its solubility be if its partial pressure over water were 107 kPa? The vapor pressure of water is 3.0 kPa at 25°C. Example 5:

29 P total = P H2O + P O2 P O2 = P total - P H2O = = 98 kPa Solution: