1. Water and Aqueous Systems
Chapter 15
Water and Aqueous Systems

2. Water and its Properties
Water is a polar molecule
Has hydrogen bonding
This creates:
High surface tension
Low vapor pressure

3. Surface Tension
The inward force, or pull, that tends to minimize the surface area of the liquid

5. Water Strider

6. Surfactant
Any substance that interferes with the hydrogen bonding between water molecules
Reduces surface tension
Examples: soaps, detergents

7. Water and its Properties
Vapor Pressure
Is the result of molecules escaping from the surface of the liquid and entering the vapor phase

8. Vapor Pressure of Purell™

9. Water and its Properties
Water in the Solid State
Ice is less dense than water
Structure of ice is a regular open framework

10. Homogeneous Aqueous Systems
Aqueous Solution – water that contains dissolved substances
Solvent – the dissolving medium (water)
Salt is the solute (gets dissolved or dispersed in the water)
NaCl (s) + H2O (l) → NaCl (aq)
solute solvent solution

11. Figure: 13-01

12. Electrolytes
A compound that conducts electric current when in aqueous solution or in the molten state
All ionic compounds are electrolytes because they dissociate into ions
Strong electrolytes – compounds dissociate completely as separate ions
Weak electrolytes – conducts electricity poorly because ions/molecules do not dissociate completely

14. Figure: 13-09

15. Figure: 13-26

16. Nonelectrolyte
A compound that does not conduct an electric current in wither aqueous solution or molten state
Examples: molecular compounds (covalent bonds)

17. Figure: 13-31

18. Sample Exercise
Which of the following substances dissolve appreciably in water?
HCl
NH3
CH4
NaI
MgSO4
CaCO3

19. Sample Exercise
Decide which compounds are more likely to dissolve in water and which are more likely to dissolve in gasoline
CCl4
Na2SO4
Methane (CH4)
KCl

20. Sample Exercise
Calculate the percent by mass of water in washing soda, sodium carbonate decahydrate (Na2CO3  10H2O)
Answer: 62.9%

21. Practice Problems What is the percent by mass of water in CuSO45H2O?
Answer: 36%

22. Practice Problems
Calculate the percent by mass of water in calcium chloride hexahydrate (CaCl26H2O)?
Answer: 49.5%

23. Sample Exercise
Write equations to show how these substances ionize or dissociate in water
NH4Cl
Cu(NO3)2
HC2H3O2
BaCl2

24. Chapter 16
Solutions

25. Solution Formation How quickly the solute dissolves in a solvent
Rate can be increased by increasing:
Agitation (stirring)
Temperature
Surface Area

26. Agitation
Mixing the solute increases the chance for the solvent to react and form a solution

27. 2. Temperature
At high temperatures, the kinetic energy of molecules is greater than at lower temperatures
The more rapid motion of molecules leads to an increase in frequency and force in collisions between molecules

28. 3. Surface Area
Increasing the surface area of a solute increases the chances for a reaction

29. Solubility
Is the amount of substance that dissolves in a given quantity of a solvent at a given temperature to produce a saturated solution
Solute + solvent  solution
Ex: NaCl(s)(solute) + water(l)(solvent)  NaCl(aq) (solution)
Saturated solution – contains the maximum amount of solvent at constant temperature
Unsaturated – a solution that contains less solute than a saturated solution

30. Miscible – two liquids that dissolve in each other
Immiscible – liquids that are insoluble in one another

31. Factors Affecting Solubility
Temperature and Pressure
Temperature
Affects the solubility of solids, liquids, and gaseous solutes in a solvent
Pressure
Affects only the solubility of gaseous solutes

32. 1. Temperature Table G – Solubility Curve
The solubility of most solid substances increases as temperature increases (proportional)
Gases are more soluble in liquids when temperatures are low (inverse)
Supersaturated solution – contains more solute that it can theoretically hold at a given temperature

34. Sample Exercise
At what temperature will the solubility of KNO3 be 50 g/per 100g of H2O? At 100 g of KNO3?
How much KClO3 is needed to saturate 50 g of H2O at 90C?
How many grams of NaNO3 will precipitate if a saturated solution of NaNO3 in 200g H2O at 50°C is cooled to 20°C?

35. Sample Exercise
The solubility of KCl in water is 34.0 g KCl/100g H2O at 20°C. A warm solution containing 50.0 g KCl in 100 g H2O is cooled from 75°C to 20°C.
How many grams of KCl remain dissolved?
How many grams came out of solution?

36. Sample Exercise
A solution contains 70 grams/100 g H2O at 60°C. Determine if the solution is supersaturated, saturated, or unsaturated for: KI, NaNO3, HCl, KNO3
Which solution has the highest solubility in 100 g of H2O at 40°C?

37. Sample Exercise
A solution contains 14 g of KCl in 100 g of water at 40°C. What is the minimum amount of KCl that must be added to make this a saturated solution?
How many grams of the compound KCl must be dissolved in 200 g of water to make a saturated solution at 60°C

38. Sample Exercise
Which amount of the compound dissolved in 100 g of water at the stated temperature represents a solution that is saturated?
20 g KClO3 at 80°C
40 g KNO3 at 25°C
40 g KCl at 60°C
60 g NaNO3 at 40°C

39. 2. Pressure Increasing pressure prevents vaporization
Increasing pressure makes gaseous molecules more soluble in liquid form

40. Concentrations of Solutions
Molarity (M) - The number of moles of a solute dissolved in 1L of solution
Also known as molar concentration
number of moles of solute
Molarity (M) = number of liters of solution
Table T

41. Dilute Solution – one that contains a small amount of solute
Concentrated Solution – contains a large amount of solute

42. Sample Problem
Household laundry bleach is a dilute aqueous solution of sodium hypochlorite (NaClO). How many moles of solute are present in 1.5 L of 0.70 M NaClO?
Answer: 1.1 mol NaClO

43. Practice Problems
How many moles of ammonium nitrate are in 335 mL of M NH4NO3?
Answer: moles
How many moles of solute are in 250 mL of 2.0 M CaCl2? How many grams of CaCl2 is this?
Answer: 55 grams

44. Practice Problems
A saline solution contains 0.90g of NaCl per mL of solution. What is its molarity?
Answer: mol/L
How many grams of solute are present in 1.5 L of 0.20M Na2SO4?
Answer: 42.6 grams

45. Making Dilutions
Making a solution less concentrated by diluting it with more solvent
M1 x V1 = M2 x V2
Volumes can be in liters or in milliliters as long as the same units are used for both V1 and V2

46. Sample Problem
How many milliliters of aqueous 2.00 M MgSO4 solution must be diluted with water to prepare mL of aqueous M MgSO4?
Answer: 20.0 mL

47. Sample Exercise
How many milliliters of a solution of 4.00 M KI are needed to prepare mL of M KI?
Answer: 47.5 mL
How could you prepare 250 mL of 0.20 M NaCl using only a solution of 1.0 M NaCl and water?
Answer: 50 mL

48. Parts per Million
Ratio between mass of solute and total mass of solution
Useful for very dilute solutions
Table T
This method of reporting concentrations is useful for extremely dilute solutions when molarity and percent mass would be difficult to interpret. For example, chlorine is used as a disinfectant in swimming pools. Only about 2g of chlorine per 1,000,000g of water is necessary to keep the pool sanitized. Finding molarity and percent mass would result in numbers too small to be useful. Parts per million is often used to report a measured amount of air or water pollutants.

49. Sample Exercises
Carbon dioxide gas has a solubility of 0.972g in 100.0g of H2O at 40°C, express this in parts per million.
ppm = grams of solute x 1,000,000
grams of solution
Approximately g of oxygen can be dissolved in 100.0g of water at 20oC. Express this in terms of parts per million.
0.972g x 1,000,000 = g
9626
9630ppm CO2
0.0043g x 1,000,000 = g
42.99
43ppm O2

50. Colligative Properties of Solutions
Colligative property -
A property that depends on the number of solute particles, not their identity
Three important examples in solutions:
Vapor-pressure lowering
Freezing-point depression
Boiling-point elevation

51. Vapor-Pressure Lowering
a measure of the force exerted by a gas above a liquid
Solutions with nonvolatile (not easily vaporized) solutes have lower vapor pressures than pure solvents
E.x. Salt water has a lower vapor pressure than pure water
The vapor pressure of a solution of a nonvolatile solute is less than the vapor pressure of a pure solvent.
In a pure solvent, equilibrium is established between the liquid and the vapor.
B. In a solution, solute particles reduce the number o free solvent particles able to escape the liquid. Equilibrium is established at a lower vapor pressure.

52. Vapor-Pressure Lowering
Water forms a “shell” around dissolved particles
This leaves fewer water molecules with enough KE to escape as vapor
The decrease in a solution’s vapor pressure is proportional to the number of particles the solute makes in a solution
E.x. NaCl is 2x as effective as glucose, CaCl2 is 3x as effective as glucose
Particle concentrations differ for dissolved covalent and ionic compounds in water.
Three moles of glucose dissolved in water produce 3 mol of particles because glucose does not dissociate.
B. Three moles of sodium chloride dissolved in water produce 6 mol of particles because each formula unit of NaCl dissociates into two ions.
Three moles of calcium chloride dissolved in water produce 9 mil of particles because each formula unit of CaCl2 dissociates into three ions.

53. Freezing-Point Depression
difference in temperature between the freezing point of a solution and the freezing point of the pure solvent
Solutions have lower freezing-points than pure solvents

54. Freezing-Point Depression
The presence of a solute disrupts the formation of the orderly pattern found in a solid
Therefore, more KE must be removed before the solution can freeze
The magnitude of the freezing-point depression is proportional to the number of solute particles dissolved in the solvent
Why would CaCl2 be better than NaCl for melting ice?
NaCl produces 2 ions
CaCl2 produces 3 ions
Ethylene glycol, antifreeze, is added to the water in automobile cooling systems to depress the freezing point of water below 0oC. Automobiles can thus withstand subfreezing temperatures without freezing up.

55. Boiling-Point Elevation
Boiling point – temperature at which the vapor pressure of liquid equals the atmospheric pressure
Boiling-point elevation – the difference in temperature between the boiling point of a solution and the boiling point of the pure solvent

56. Boiling-Point Elevation
Solutions have higher boiling-points than pure solvents
Recall… nonvolatile solids lower the vapor pressure
Therefore, it takes more KE for the solvent particles to overcome the attractive forces that keep them in the liquid
The magnitude of the boiling-point elevation is proportional to the number of solute particles dissolved in the solvent
Elevating the boiling point of a solution has some familiar applications. For example, to make fudge, a lot of sugar and some flavorings are mixed with water and the solution is boiled. As the water slowly boils away, the concentration of sugar in the solution increases. As the concentration increases, the boiling point steadily rises. As you watch the temperature on a candy thermometer rise, you know that the solution is becoming more concentrated.

57. Sample Exercises
An equal number of moles of KI and MgF2 are dissolved in equal volumes of water. Which solution has the higher:
Boiling point -
Vapor pressure -
Freezing point -
Why is salt, NaCl, put on icy roads and sidewalks in the winter?
MgF2 solution
KI solution
KI solution
The addition of NaCl lowers the temperature at which water freezes.

58. Assume equal aqueous concentrations of each of the following substances. Which has the lowest freezing point?
C2H22O6
CH3OH
C12H22O11
NaOH
NaCl produces the most particles
upon dissolving.
Freezing point depression is a colligative property that depends on the number of solute particles in a solvent.

59. Which solution has the highest boiling point? seawater 1.0M KNO3 or
0.100M KCl or
How does the freezing point of a 1.5M solution of potassium nitrate compare to a 1.5M solution of sodium nitrate?
or distilled water
1.5M KNO3
0.100M MgCl2
Both solutions are effectively the same since they
both produce 2 ions; there is no difference in their
freezing points.

60. Vapor Pressure & Boiling
Liquids are held together by weak forces
Some particles at the surface of the liquid have sufficient energy to escape & enter the gaseous phase
They vaporize!
Vapor pressure = the pressure that these gaseous particles exert on the liquid below
What will happen to the vapor pressure as the temperature is increased?
As more particles gain more KE, they can escape the liquid, became a gas & exert pressure
It increases!

61. As temperature ↑, vapor pressure ↑
At some point the
vapor pressure = atmospheric pressure
Gases vaporize not only at the surface, but also within the liquid (bubbles)
This is the boiling point -
When the vapor pressure is equal to the external pressure

62. Of the substances shown on the graph, propanone exerts the most pressure, about 93 kPa at a temperature of 50oC. It can be inferred that propanone has the weakest intermolecular forces holding it in the liquid phase, while ethanoic acid has the greatest, exerting only about 8 kPa of pressure.
The normal boiling point of water is 100oC. At this temperature, the vapor pressure of water is kPa. The line representing kPa on Table H shows the normal boiling point of ethanol to be 78oC. When the pressure is less than kPa, the boiling point will be less than the normal value. Water will boil at about 70oC when the pressure is about 30 kPa. If the pressure is greater than normal, liquids will boil at temperatures above their normal boiling points. When atmospheric pressure is about 145 kPa, water boils at 110oC.

63. Sample Exercises What is the vapor pressure of water at 105°C?
What is the vapor pressure for a sample of ethanoic acid at 100°C?
Which of the substances on Table H has the greatest intermolecular forces of attraction between molecules?
Which of the substances on Table H has the weakest intermolecular forces of attraction?
120 kPa
58 kPa
Ethanoic acid
Propanone

64. What is the normal boiling point for water?
What is the vapor pressure of water at its normal boiling point?
At what temperature will water boil at a pressure of 30 kPa?
At what temperature will water boil at a pressure of 145 kPa?
As the pressure on a liquid is changed from 100 kPa to 120 kPa, the temperature at which the liquid will boil will be?
100oC
101.3 kPa
70oC
110oC
Increased