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CHAPTER 4. First year . Solutions B By Dr. Hisham Ezzat 2011- 2012 . Example 6.

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CHAPTER 4

First year

• Solutions

B

By

Dr. HishamEzzat

2011- 2012

Example 6
• Calculate the molarity and the molality of an aqueous solution that is 10.0% glucose, C6H12O6. The density of the solution is 1.04 g/mL. 10.0% glucose solution has several medical uses. 1 mol C6H12O6 = 180 g
Calculate the molality and the molarity of an aqueous solution that is 10.0% glucose, C6H12O6. The density of the solution is 1.04 g/mL. 10.0% glucose solution has several medical uses. 1 mol C6H12O6 = 180 g

You calculate the molarity!

Example 7
• Calculate the molality of a solution that contains 7.25 g of benzoic acid C6H5COOH, in 2.00 x 102mL of benzene, C6H6. The density of benzene is 0.879 g/mL. 1 mol C6H5COOH = 122 g

You do it!

Example 8
• What are the mole fractions of glucose and water in a 10.0% glucose solution (Example 6)?

You do it!

Factors Affecting Solubility

• 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).

Chapter 13

Factors Affecting Solubility

Solute-Solvent Interactions

• “Like Dissolves Like”
• Polar molecules will dissolve in polar solvents.
• Non-polar molecules will dissolve in non-polar solvents.
• 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.
(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.
(Miscibility
• For example, methanol, CH3OH, is very soluble in water
(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.
(Miscibility)
• Nonpolar molecules essentially “slide” in between each other. تنزلق
• For example, carbon tetrachloride and benzene are very miscible.
Effect of Temperature on Solubility

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., Ce2(SO4)3).

Gases are less soluble at higher temperatures. An environmental application of this is thermal pollution.

Solvent

DHsolvent

Solute

DHsolute

DHsolution

Solution

exothermic

Solute + solvent → solution + heat

or

endothermic

Solute +solvent + heat → solution

ΔH = H solution - (H solute + H solvent).

ΔH solutionnegative = exothermic

positive. = endothermic,

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:

Heating or cooling the system.

Changing the pressure of the system.

Changing the concentrations of reactants or products.

Effect of Temperature on Solubility
Example of endothermic dissolution

21 kJ + KI(s)  K+ + I-

LeChatelier's principle

• 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.

the solubility of KI increases with increasing temperature.

Example of exothermic dissolution

solubility of lithium iodide decreases with an increase in temperature

LiI (s)  Li+ + I- + 71 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.)

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.

Factors Affecting Solubility

Pressure Effects

• Solubility of a gas in a liquid is a function of the pressure of the gas.
• pressure, solubility.

Factors Affecting Solubility

Pressure Effects

• Henry’s Law – The solubility of a gas increases in direct proportion to its partial pressure above the solution.
• Cg - solubility of gas
• Pg - the partial pressure of the gas
• k - Henry’s law constant.

Carbonated beverages are bottled under PCO2> 1 atm. As the bottle is opened, PCO2 decreases and the solubility of CO2 decreases. Therefore, bubbles of CO2 escape from solution.

Problem:

At 740 torr and 20°C, nitrogen has solubility in H2O of 0.018 g /I. At 620 torr and 20°C its solubility is 0.015 g/l.

Do these data show that nitrogen obey Henry's law or not?

Example 5:

At 25°C oxygen gas collected over water at a total pressure of 101 kPa is soluble to the extent of 0.0393 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.

Solution:

P total = PH2O + PO2

PO2 = P total - PH2O = 101-3 = 98 kPa