Chapter 15

Water and Aqueous Systems

Water and its Properties

Water is a polar molecule

Has hydrogen bonding

This creates:

High surface tension

Low vapor pressure

Surface Tension

The inward force, or pull, that tends to minimize the surface area of the liquid

Water Strider

Surfactant

Any substance that interferes with the hydrogen bonding between water molecules

Reduces surface tension Examples: soaps, detergents

Water and its Properties

Vapor PressureIs the result of molecules escaping from the surface of the liquid and entering the vapor phase

Vapor Pressure of Purell™

Water and its Properties

Water in the Solid State•Ice is less dense than water•Structure of ice is a regular open framework

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

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

Nonelectrolyte

A compound that does not conduct an electric current in wither aqueous solution or molten state

Examples: molecular compounds (covalent bonds)

Sample Exercise

Which of the following substances dissolve appreciably in water?

HCl NH3

CH4

NaI MgSO4

CaCO3

Sample Exercise

Decide which compounds are more likely to dissolve in water and which are more likely to dissolve in gasoline

a. CCl4b. Na2SO4

c. Methane (CH4)

d. KCl

Sample Exercise

Calculate the percent by mass of water in washing soda, sodium carbonate decahydrate (Na2CO3 10H2O)

Answer: 62.9%

Practice Problems

What is the percent by mass of water in CuSO45H2O?

Answer: 36%

Practice Problems

Calculate the percent by mass of water in calcium chloride hexahydrate (CaCl26H2O)?

Answer: 49.5%

Sample Exercise

Write equations to show how these substances ionize or dissociate in water

a. NH4Cl

b. Cu(NO3)2

c. HC2H3O2

d. BaCl2

Chapter 16

Solutions

Solution Formation

How quickly the solute dissolves in a solvent

Rate can be increased by increasing:

1. Agitation (stirring)

2. Temperature

3. Surface Area

1. Agitation

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

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

3. Surface Area

Increasing the surface area of a solute increases the chances for a reaction

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

Miscible – two liquids that dissolve in each other

Immiscible – liquids that are insoluble in one another

Factors Affecting Solubility

Temperature and Pressure

1. Temperature Affects the solubility of solids, liquids, and

gaseous solutes in a solvent

2. Pressure Affects only the solubility of gaseous

solutes

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

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 90C?

How many grams of NaNO3 will precipitate if a saturated solution of NaNO3 in 200g H2O at 50°C is cooled to 20°C?

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.

a. How many grams of KCl remain dissolved?

b. How many grams came out of solution?

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?

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

Sample Exercise

Which amount of the compound dissolved in 100 g of water at the stated temperature represents a solution that is saturated?

(1)20 g KClO3 at 80°C

(2)40 g KNO3 at 25°C

(3)40 g KCl at 60°C

(4)60 g NaNO3 at 40°C

2. Pressure

Increasing pressure prevents vaporization

Increasing pressure makes gaseous molecules more soluble in liquid form

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

Dilute Solution – one that contains a small amount of solute

Concentrated Solution – contains a large amount of solute

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

Practice Problems

How many moles of ammonium nitrate are in 335 mL of 0.425 M NH4NO3?

Answer: 0.142 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

Practice Problems

A saline solution contains 0.90g of NaCl per 100.0 mL of solution. What is its molarity?

Answer: 0.155 mol/L How many grams of solute are present in 1.5 L

of 0.20M Na2SO4? Answer: 42.6 grams

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

Sample Problem

How many milliliters of aqueous 2.00 M MgSO4 solution must be diluted with water to prepare 100.0 mL of aqueous 0.400 M MgSO4?

Answer: 20.0 mL

Sample Exercise

How many milliliters of a solution of 4.00 M KI are needed to prepare 250.0 mL of 0.760 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

Parts per Million

Ratio between mass of solute and total mass of solution

Useful for very dilute solutions Table T

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 0.0043g of oxygen can be dissolved in 100.0g of water at 20oC. Express this in terms of parts per million.

ppm = grams of solute x 1,000,000

grams of solution

0.972g x 1,000,000 =100.972g

9630ppm CO2

9626

0.0043g x 1,000,000 = 100.0043g

42.99

43ppm O2

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 depressionBoiling-point elevation

Vapor-Pressure Lowering

Vapor Pressure – 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

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

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

Freezing-point depression –

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

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

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

Sample Exercises

An equal number of moles of KI and MgF2 are dissolved in equal volumes of water. Which solution has the higher:

a) Boiling point -

b) Vapor pressure -

c) 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.

Assume equal aqueous concentrations of each of the following substances. Which has the lowest freezing point?

a) C2H22O6

b) CH3OH

c) C12H22O11

d) NaOH

NaCl produces the most particlesupon dissolving. Freezing point depression is a colligative property that depends on the number of solute particles in a solvent.

Which solution has the highest boiling point?

a) seawater

b) 1.0M KNO3 or

c) 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.

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!

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

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?

58 kPa

Propanone

120 kPa

Ethanoic acid

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? Increased

100oC

101.3 kPa

70oC

110oC