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CHAPTER 4

Solutions

BBy

Dr. Hisham Ezzat

2011- 2012

First year

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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

? mol C H O

kg H O

g C H O

90.0 g H O 6 12 6

2

6 12 6

2

10 0.

OH kg 1

OH g 1000

OH g 90.0

OHC g 0.10

OH kg

OHC mol ?

2

2

2

6126

2

6126

.molalityin ion concentrat theis This

OHC 617.0OHC g 180

OHC mol 1

OH kg 1

OH g 1000

OH g 90.0

OHC g 0.10

OH kg

OHC mol ?

6126

6126

6126

2

2

2

6126

2

6126

m

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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!

61266126

6126

6126

2

6126

OHC 578.0OHC g 180

OHC mol 1

L 1

mL 1000

nsol' mL

nsol' g 04.1

n sol' g 100.0

OHC g 0.10

OH L

OHC mol ?

M

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Example 7

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

You do it!You do it!

? mol C H COOH

kg C H

g C H COOH

200.0 mL C H

mL C H

0.879 g C H

g C H

1 kg C H

mol C H COOH

122 g C H COOH C H COOH

6 5

6 6

6 5

6 6

6 6

6 6

6 6

6 6

6 5

6 56 5

7 25 1

1000 10 338

.

. m

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Example 8 What are the mole fractions of glucose

and water in a 10.0% glucose solution (Example 6)?

You do it!You do it!

6126

6126

6126

61266126

2

OHC mol 0556.0OHC g 180

OHC mol 1

OHC g 0.10OHC mol ?

water.of g 90.0 and glucose of g 10.0

are heresolution t thisof g 101.00In

6

OH mol 00.5OH g 18

OH mol 1OH g 0.90OH mol ? 2

2

222

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Now we can calculate the mole fractions.

011.0989.000.1

011.0

OHC mol 0.0556 +OH mol 00.5

OHC mol 0556.0

989.0

OHC mol 0.0556 +OH mol 00.5

OH mol 00.5

61262

6126OHC

61262

2OH

6126

2

X

X

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.

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(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.

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(Miscibility

For example, methanol, CH3OH, is very soluble in water

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(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.

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(Miscibility)

Nonpolar molecules essentially “slide” in between each other. تنزلق For example, carbon tetrachloride and benzene

are very miscible.

C Cl

Cl

Cl

Cl

C

CC

C

CC

H

H

H

H

H

H

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.

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Effect of Temperature on Solubility

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SolventSolvent

SolutionSolution

HHsolventsolvent HHsolutesolute

solutionsolution

SoluteSolute

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exothermicSolute + solvent → solution + heat

or

endothermic Solute + solvent + heat → solution

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

ΔH solution negative = exothermic

positive. = endothermic,

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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-

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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.

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Temperature EffectsTemperature Effects

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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.)

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Factors Affecting SolubilityFactors Affecting SolubilityTemperature EffectsTemperature 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.

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Factors Affecting SolubilityFactors Affecting Solubility

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.

ggkPC

Pressure Effects

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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.

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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?

Problem:

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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.

Example 5:

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P total = PH2O + PO2

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

Solution: