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Chapter 14

Solutions

I have no doubt that in reality the future will be vastly more surprising than anything I can imagine. Now my own

suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose.

John Burdon Sanderson Haldane

Outline

I. Solutions

II. Units of Concentration III. Properties of Solutions IV. Solubility

V. Reactions

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2

Solute and Solvent

Solutions

• are homogeneous

mixtures of two or more substances

• contain a solute,

usually in smaller quantity

• contain a solvent,

(3)

Solution: a homogeneous mixture of one or more solutes and a solvent.

Solvent: dissolving agent that is usually the most abundant substance in the mixture.

Note: a solution does not always just refer to liquids.

Example: Air is a solution composed of N2, O2, Ar and CO2

N2 is the solvent as it composes 78% of air. Solute: substance being dissolved.

General Properties of Solutions

(4)

Solubility: the amount of a substance that will dissolve in a specific amount of solvent at a given temperature.

Example 27 g KBr/100g H2O at 23 ºC

Miscible: when two liquids dissolve in each other.

Immiscible: when two liquids do not dissolve one another.

A mixture of oil and water is

immiscible.

Solubility

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Formation of a Solution

Na+ and Cl ions

on the surface of a

NaCl crystal are attracted to polar water molecules

in solution are

hydrated as several H2O molecules

(6)

Attractions in Solutions

Interactions between solutes and solvents that

are similar in polarity will form a solution are not similar will not form a solution

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7

When NaCl(s) dissolves in water, the reaction can be written as

H2O

NaCl(s) Na+(aq) + Cl(aq)

solid separation of ions

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Two substances form a solution

when there is an attraction between the

particles of the solute and solvent

when a polar solvent such as water dissolves

polar solutes such as sugar and ionic solutes such as NaCl

when a nonpolar solvent such as hexane

(C6H14) dissolves nonpolar solutes such as oil or grease

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Like Dissolves Like

Solvents Solutes

Water (polar) Ni(NO3)2

CH2Cl2(nonpolar) (polar)

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Will the following solutes dissolve in water? Why? 1) Na2SO4

2) gasoline (nonpolar) 3) I2

4) HCl

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In water, strong electrolytes

• completely dissociate in water • produce ions

• conduct an electric current

Examples:

H2O 100% ions

NaCl(s) Na+(aq) + Cl− (aq)

H2O

CaBr2(s) Ca2+(aq) + 2Br−(aq)

Solutes: Strong Electrolytes

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In water, weak electrolytes

• dissolve mostly as molecules • produce a few ions

• conduct a weak current

Examples:

HF(g) + H2O(l)

H3O+(aq) + F −(aq)

NH3(g) + H2O(l)

NH4+(aq) + OH− (aq)

Solutes: Weak Electrolytes

The weak electrolyte in an

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In water, nonelectrolytes

• dissolve as molecules • do not produce ions

• do not conduct a current

Example:

C12H22O11(s) C12H22O11(aq)

Solutes: Nonelectrolytes

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Solutes in Aqueous Solutions

(15)

Na+, K+, NH 4+

Cl-, Br-, I- Ag+, Hg

22+, Pb2+

Soluble Insoluble

Nitrates (NO3-)

Acetates, (C2H3O2-)

Sulfates (SO42-),

Ag+, Ca2+ (slightly)

NH4+, Group I

Except

Except

Except

Ba2+, Sr2+, Pb2+

Carbonates (CO32-)

Phosphates (PO43-)

OH-, Sulfides (S2-)

Solubility Rules

(16)

Predict the solubility of barium sulfate.

a. soluble b. insoluble

Predict the solubility of NaCl.

a. soluble b. insoluble

Most sulfates are soluble, except Ba2+.

All Na+ salts are soluble.

Solutions Practice

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a. soluble b. insoluble

Predict the solubility of silver nitrate.

a. soluble b. insoluble

Predict the solubility of silver hydroxide.

a. soluble b. insoluble

Predict the solubility of ammonium carbonate. All NO3- salts are soluble.

Most hydroxides are insoluble.

All NH4+ salts are soluble.

© 2014 John Wiley & Sons, Inc. All rights reserved.

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Equations for Forming Solids

A chemical equation shows the complete formulas of all the compounds in a reaction.

Pb(NO3)2(aq) + 2NaCl(aq) PbCl2(s) + 2NaNO3(aq)

An ionic equation shows the ionic compounds as separate ions.

Pb2+(aq) + 2NO

3−(aq) + 2Na+(aq) + 2Cl−(aq)

PbCl2(s) + 2Na+(aq) + 2NO

3−(aq)

A net ionic equation shows only the ions that react to form a solid.

Pb2+(aq) + 2Cl−(aq) PbCl

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Example of Writing Equations for an

Insoluble Salt

Write the net ionic equation for the reaction of Ba(NO3)2(aq) + Na2CO3(aq)

STEP 1 Predict the products of the reaction and determine their phase (s), (l), (g), (aq)

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Equations for an Insoluble Salt

(continued)

STEP 3 Write the ionic equation including any solid.

Ba2+(aq) + 2NO

3−(aq) + 2Na+(aq) + CO32 −(aq)

BaCO3(s) + 2Na+(aq) + 2NO

3−(aq))

STEP 4 Write the net ionic equation. Cross out the spectator ions in the ionic equation.

Ba2+(aq) + 2NO

3−(aq) + 2Na+(aq) + CO32 −(aq)

BaCO3(s) + 2Na+(aq) + 2NO

3−(aq))

Ba2+(aq) + CO

32−(aq) BaCO3(s)

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Learning Check

Write the formula of any insoluble salt, if any. Write the net ionic equation for any of the

following which form insoluble salts: A. BaCl2(aq) + Na2SO4(aq)

B. AgNO3(aq) + KCl(aq) C. KNO3(aq) + NaCl(aq)

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Example – Conventional, Total Ionic and

Net Ionic Equations

Zinc metal reacts with aqueous copper(II)

(23)

Example – Conventional, Total Ionic and

Net Ionic Equations

Aqueous ammonium nitrate reacts with aqueous

sodium hydroxide to produce aqueous sodium

nitrate and ammonia gas and water.

What is the total ionic equation?

A. NH

4

NO

3 (aq)

+ NaOH

(aq)

 NaNO

3 (aq)

+ NH

3 (g)

+ H

2

O

(l) B. NH4 +

(aq) + NO3- (aq) + Na+ (aq) + OH- (aq)  Na+ (aq) + NO3- (aq) + NH3 (g) + H2O (l)

C. NH

4 +

(aq)

+ OH-

(aq)

 NH

3 (g)

+ H

2

O

(l)

D. All spectator ions

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24

Solubility

is the maximum amount of solute that dissolves

in a specific amount of solvent

can be expressed as grams of solute in 100

grams of solvent, usually water g of solute

100 g water

(25)

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Comparing Unsaturated and

Saturated Solutions

(26)

There are limits to the solubility of a compound at a given temperature.

Saturated solutions: contain the maximum amount of dissolved solute in a solvent.

Saturated solutions are still dynamic; dissolved solute is in equilibrium with undissolved solute.

Unsaturated solutions: contain less than the maximum amount of possible dissolved solute in a solvent.

undissolved solute dissolved solute

Saturated and Unsaturated

Solutions

(27)

Supersaturated solutions: contain more solute than needed to saturate a solution at a given temperature.

These solutions are unstable -- disturbing the solutions can cause precipitation of solute.

How is this possible?

Heating a solution can allow more to dissolve. Upon cooling to ambient temperature,

the solution is supersaturated.

Some hotpacks release heat by crystallization of a supersaturated solution of sodium acetate.

Supersaturated Solutions

(28)

Example - Solutions

What type of

solution is sodium acetate at 75 °C and 50g/100 g H2O?

A. Saturated B. Unsaturated C. Supersaturated D. Not enough

(29)

What happens to a supersaturated solution if more solute is added?

(30)

30

Effect of Temperature on Solubility

Solubility

• depends on

temperature

• of most solids

increases as

temperature increases

• of gases decreases as

(31)

Effect of Agitation/Stirring

Stirring a solution briskly breaks up a solid into smaller pieces, increasing surface area,

thereby increasing the rate of dissolution.

© 2014 John Wiley & Sons, Inc. All rights reserved.

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32

Solubility and Pressure

Henry’s law states

the solubility of a

gas in a liquid is directly related to the pressure of that gas above the liquid

at higher pressures,

(33)

33

A. Why could a bottle of carbonated drink possibly burst (explode) when it is left out in the hot sun?

B. Why do fish die in water that is too warm?

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34

The concentration of a solution

is the amount of solute dissolved in a specific

amount of solution amount of solute amount of solution

as percent concentration describes the amount

of solute that is dissolved in 100 parts of solution

amount of solute 100 parts solution

(35)

35

The mass percent (m/m)

• concentration is the percent by mass of solute in a

solution

Mass percent (m/m) =

mass of solute x 100% mass of solute + mass of solvent

• is the g of solute in exactly100 g of solution

Mass percent = g of solute x 100% 100. g of solution

(36)

36

Mass percent (%m/m) is calculated from the

grams of solute (g KCl) and the grams of solution (g KCl solution).

g of KCl = 8.00 g

g of solvent (water) = 42.00 g

g of KCl solution = 50.00 g

8.00 g KCl (solute) x 100 = 16.0% (m/m) 50.00 g KCl solution

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37

(38)

38

A solution is prepared by mixing 15.0 g of

Na2CO3 and 235 g of H2O. Calculate the mass percent (m/m) of the solution.

(39)

Example – Mass Percent

A solution consists of 31.7 g of AgNO

3

in

(40)

Example – Mass Percent

How much of a 25.0% glucose solution is

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41

The volume percent (v/v) is

percent volume (mL) of solute (liquid) to volume

(mL) of solution

Volume percent(v/v) = mL of solute x 100% mL of solution

solute (mL) in 100 mL of solution

Volume percent (v/v) = mL of solute x 100% 100 mL of solution

(42)

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A volume percent (v/v)

solution is prepared by

using a specific volume

(mL) of solute (liquid)

adding water to the

volume line

(43)

43

Percent Conversion Factors

Two conversion factors can be written

(44)

44

How many grams of NaCl are needed to prepare

225 g of a 10.0% (m/m) NaCl solution?

How many grams of NaOH are needed to prepare

75.0 g of a 14.0% (m/m) NaOH solution?

How many milliliters of a 5.75% (v/v) ethanol solution

can be prepared from 2.25 mL of ethanol?

(45)

Saline is a 0.9 m/v % NaCl solution. What mass of sodium chloride is needed to make 50 mL of saline?

= 0.45 g NaCl

Knowns 50.0 mL solution (solution volume) 0.90 m/v% (mass/volume %)

Calculate

Solve for mass of solute (NaCl)

50.0 x 0.90 100

mass solute =

Formula

g solute

mL solution x 100

m/v % = mass solute = m/v % x mL soln100

Mass-Volume Percent

References

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