The Chemistry of Acids and Bases

1
Chemistry and Chemical Reactivity 6th
Edition John C. Kotz Paul M. Treichel
Gabriela C. Weaver
CHAPTER 17 Principles of Reactivity Chemistry
of Acids and Bases
Lecture written by John Kotz as modified by
George Rhodes
2
Chapter Goals p-837

• Use the BrÖnsted-Lowry and Lewis theories of
  acids and bases
• Apply the principles of chemical equilibrium to
  acids and bases in aqueous solution
• Predict the outcomes of reactions of acids and
  bases
• Understand the influence of structure and bonding
  on acid-base properties

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Acids and Bases

• In Ch17 we discuss acid base theories (2), the
  concept of equilibria in aqueous solution and
  their reactions.
• In Ch18 we will address the common ion effect,
  buffer solutions, acid base titrations and the
  solubility of salts

4
The Chemistry of Acids and Bases
5
Acid and Bases
6
Acid and Bases
7
Acid and Bases
Some foods contain toxic acids. Rhubarb contains
oxalic acid, a poison to the human digestive
system
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Strong and Weak Acids/Bases

• Generally divide acids and bases into STRONG or
  WEAK ones.
• STRONG ACID HNO3(aq) H2O(liq)
  ---gt H3O(aq) NO3-(aq)
• HNO3 is about 100 dissociated in water.

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Strong and Weak Acids/Bases
HNO3, HCl, H2SO4 and HClO4 are among the only
known strong acids.
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Strong and Weak Acids/Bases

• Weak acids are much less than 100 ionized in
  water.
• One of the best known is acetic acid CH3CO2H

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Strong and Weak Acids/Bases

• Strong Base 100 dissociated in water.
• NaOH(aq) ---gt Na(aq) OH-(aq)

Other common strong bases include KOH and
Ca(OH)2. CaO (lime) H2O --gt Ca(OH)2
(slaked lime)
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Strong and Weak Acids/Bases

• Weak base less than 100 ionized in water
• One of the best known weak bases is ammonia
• NH3(aq) H2O(liq) ? NH4(aq) OH-(aq)

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ACID-BASE THEORIES

• The most general theory for common aqueous acids
  and bases is the BRØNSTED - LOWRY theory
• ACIDS DONATE H IONS
• BASES ACCEPT H IONS

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ACID-BASE Behavior

• Molecules or ions that can alternately behave as
  either a BRØNSTED - LOWRY acid or base are called
  amphiprotic
• ACIDS DONATE H IONS
• BASES ACCEPT H IONS

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ACID-BASE THEORIES

• The Brønsted definition means NH3 is a BASE in
  water and water is itself an ACID

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ACID-BASE THEORIES

• NH3 is a BASE in water and water is itself an
  ACID

NH3 / NH4 is a conjugate pair related by the
gain or loss of H Every acid has a conjugate
base - and vice-versa.
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Conjugate Pairs
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More About Water

• H2O can function as both an ACID and a BASE.
• In pure water there can be AUTOIONIZATION

Equilibrium constant for autoion Kw Kw
H3O OH- 1.00 x 10-14 at 25 oC
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More About Water
Autoionization

• Kw H3O OH- 1.00 x 10-14 at 25 oC
• In a neutral solution H3O OH-
• so Kw H3O OH-
• and so H3O OH- 1.00 x 10-7 M

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The Ion Product of Water
H2O constant
KcH2O Kw HOH-
The ion-product constant (Kw) is the product of
the molar concentrations of H and OH- ions at a
particular temperature.
Solution Is
H OH-
neutral
At 250C Kw HOH- 1.0 x 10-14
H gt OH-
acidic
H lt OH-
basic
15.2
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Calculating H3O OH-

• You add 0.0010 mol of NaOH to 1.0 L of pure
  water. Calculate H3O and OH-.
• Solution
• 2 H2O(liq) ? H3O(aq) OH-(aq)
• Le Chatelier predicts equilibrium shifts to the
  ____________.
• H3O lt 10-7 at equilibrium.
• Set up a ICE table.

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Calculating H3O OH-

• You add 0.0010 mol of NaOH to 1.0 L of pure
  water. Calculate H3O and OH-.
• Solution

2 H2O(liq) ? H3O(aq) OH-(aq)
initial 0 0.0010 change x x
equilib x 0.0010 x Kw (x) (0.0010
x) Because x ltlt 0.0010 M, assume OH-
0.0010 M H3O Kw / 0.0010 1.0 x 10-11 M
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Calculating H3O OH-

• You add 0.0010 mol of NaOH to 1.0 L of pure
  water. Calculate H3O and OH-.
• Solution
• 2 H2O(liq) ? H3O(aq) OH-(aq)
• H3O Kw / 0.0010 1.0 x 10-11 M

This solution is _________ because H3O lt
OH-
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H3O, OH- and pH

• A common way to express acidity and basicity is
  with pH
• pH log (1/ H3O) - log H3O
• In a neutral solution, H3O OH-
  1.00 x 10-7 at 25 oC
• pH -log (1.00 x 10-7) - (-7) 7

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H3O, OH- and pH

• What is the pH of the 0.0010 M NaOH
  solution?
• H3O 1.0 x 10-11 M
• pH - log (1.0 x 10-11) 11.00
• General conclusion
• Basic solution pH gt 7
• Neutral pH 7
• Acidic solution pH lt 7

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The pH Scale
Active Figure 17.2
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H3O, OH- and pH

• If the pH of Coke is 3.12, it is ____________.
• Because pH - log H3O then
• log H3O - pH
• Take antilog and get
• H3O 10-pH
• H3O 10-3.12 7.6 x 10-4 M

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pH of Common Substances
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Other pX Scales

• In general pX -log X
• and so pOH - log OH-
• Kw H3O OH- 1.00 x 10-14 at 25 oC
• Take the log of both sides
• -log (10-14) - log H3O (-log OH-)
• pKw 14 pH pOH

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Equilibria Involving Weak Acids and Bases

• Aspirin is a good example of a weak acid, Ka
  3.2 x 10-4

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Equilibria Involving Weak Acids and Bases

• Acid Conjugate Base
• acetic, CH3CO2H CH3CO2-, acetate
• ammonium, NH4 NH3, ammonia
• bicarbonate, HCO3- CO32-, carbonate
• A weak acid (or base) is one that ionizes to a
  VERY small extent (lt 5).

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Equilibria Involving Weak Acids and Bases

• Consider acetic acid, CH3CO2H (HOAc)
• HOAc H2O ? H3O OAc-
• Acid Conj. base

(K is designated Ka for ACID) Because H3O and
OAc- are SMALL, Ka ltlt 1.
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Equilibrium Constants for Weak Acids
Weak acid has Ka lt 1 Leads to small H3O and a
pH of 2 - 7
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Equilibrium Constants for Weak Bases
Weak base has Kb lt 1 Leads to small OH- and a
pH of 12 - 7
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Ionization Constants for Acids/Bases
Conjugate Bases
Acids
Increase strength
Increase strength
36
Relation of Ka, Kb, H3O and pH
37
K and Acid-Base Reactions
Reactions always go from the stronger A-B pair
(larger K) to the weaker A-B pair (smaller K).
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K and Acid-Base Reactions

• A strong acid is 100 dissociated.
• Therefore, a STRONG ACIDa good H donormust
  have a WEAK CONJUGATE BASEa poor H acceptor.
• HNO3(aq) H2O(liq) ? H3O(aq) NO3-(aq)
• STRONG A base acid
  weak B

• Every A-B reaction has two acids and two bases.
• Equilibrium always lies toward the weaker pair.
• Here K is very large.

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K and Acid-Base Reactions

• We know from experiment that HNO3 is a strong
  acid.
• 1. It is a stronger acid than H3O
• 2. H2O is a stronger base than NO3-
• 3. K for this reaction is large

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K and Acid-Base Reactions

• Acetic acid is only 0.42 ionized when HOAc
  1.0 M. It is a WEAK ACID
• HOAc H2O ? H3O OAc-
• WEAK A base acid STRONG B
• Because H3O is small, this must mean
• 1. H3O is a stronger acid than HOAc
• 2. OAc- is a stronger base than H2O
• 3. K for this reaction is small

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Types of Acid/Base Reactions

• Strong acid (HCl) Strong base (NaOH)
• H Cl- Na OH- ? H2O Na Cl-
• Net ionic equation
• H(aq) OH-(aq) ? H2O(liq)
• K 1/Kw 1 x 1014

Mixing equal molar quantities of a strong acid
and strong base produces a neutral solution.
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Types of Acid/Base Reactions

• Weak acid (acetic ac.) Strong base (NaOH)
• CH3CO2H OH- ? H2O CH3CO2-
• This is the reverse of the reaction of CH3CO2-
  (conjugate base) with H2O.
• OH- stronger base than CH3CO2-
• K 1/Kb 1/(5.6 x 10-10) 1.8 x 109

Mixing equal molar quantities of a weak acid and
strong base produces the acids conjugate base.
The solution is basic.
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Types of Acid/Base Reactions

• Strong acid (HCl) Weak base (NH3)
• H3O NH3 ? H2O NH4
• This is the reverse of the reaction of NH4
  (conjugate acid of NH3) with H2O.
• H3O stronger acid than NH4
• K 1/Ka 1.8 x 109

Mixing equal molar quantities of a strong acid
and weak base produces the basess conjugate
acid. The solution is acid.
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Types of Acid/Base Reactions

• Weak acid Weak base

• Product cation conjugate acid of weak base.
• Product anion conjugate base of weak acid.
• pH of solution depends on relative strengths of
  cation and anion.

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Types of Acid/Base Reactions Summary
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Calculations with Equilibrium Constants
0.0001 M
0.003 M

• pH of an acetic acid solution.
• What are your observations?

0.06 M
2.0 M
a pH meter, Screen 17.9
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Equilibria Involving A Weak Acid

• You have 1.00 M HOAc. Calc. the equilibrium
  concs. of HOAc, H3O, OAc-, and the pH.
• Step 1. Define equilibrium concs. in ICE table.
• HOAc H3O OAc-
• initial
• change
• equilib

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Equilibria Involving A Weak Acid

• You have 1.00 M HOAc. Calc. the equilibrium
  concs. of HOAc, H3O, OAc-, and the pH.
• Step 1. Define equilibrium concs. in ICE table.
• HOAc H3O OAc-
• initial 1.00 0 0
• change -x x x
• equilib 1.00-x x x
• Note that we neglect H3O from H2O.

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Equilibria Involving A Weak Acid
You have 1.00 M HOAc. Calc. the equilibrium
concs. of HOAc, H3O, OAc-, and the pH.

• Step 2. Write Ka expression

This is a quadratic. Solve using quadratic
formula or method of approximations (see Appendix
A).
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Equilibria Involving A Weak Acid
You have 1.00 M HOAc. Calc. the equilibrium
concs. of HOAc, H3O, OAc-, and the pH.

• Step 3. Solve Ka expression

First assume x is very small because Ka is so
small. Therefore,
Now we can more easily solve this approximate
expression.
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Equilibria Involving A Weak Acid
You have 1.00 M HOAc. Calc. the equilibrium
concs. of HOAc, H3O, OAc-, and the pH.

• Step 3. Solve Ka approximate expression

x H3O OAc- Ka 1.001/2 x
H3O OAc- 4.2 x 10-3 M pH - log H3O
-log (4.2 x 10-3) 2.37
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Equilibria Involving A Weak Acid

• Consider the approximate expression

For many weak acids H3O conj. base Ka
Co1/2 where C0 initial conc. of acid Useful
Rule of Thumb If 100Ka lt Co, then H3O
KaCo1/2
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Equilibria Involving A Weak Acid

• Calculate the pH of a 0.0010 M solution of formic
  acid, HCO2H.
• HCO2H H2O ? HCO2- H3O
• Ka 1.8 x 10-4
• Approximate solution
• H3O Ka Co1/2 4.2 x 10-4 M, pH 3.37
• Exact Solution
• H3O HCO2- 3.4 x 10-4 M
• HCO2H 0.0010 - 3.4 x 10-4 0.0007 M
• pH 3.47

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Weak Bases
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Equilibria Involving A Weak Base

• You have 0.010 M NH3. Calc. the pH.
• NH3 H2O ? NH4 OH-
• Kb 1.8 x 10-5
• Step 1. Define equilibrium concs. in ICE table
• NH3 NH4 OH-
• initial
• change
• equilib

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Equilibria Involving A Weak Base

• You have 0.010 M NH3. Calc. the pH.
• NH3 H2O ? NH4 OH-
• Kb 1.8 x 10-5
• Step 1. Define equilibrium concs. in ICE table
• NH3 NH4 OH-
• initial 0.010 0 0
• change -x x x
• equilib 0.010 - x x x

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Equilibria Involving A Weak Base

• You have 0.010 M NH3. Calc. the pH.
• NH3 H2O ? NH4 OH-
• Kb 1.8 x 10-5
• Step 2. Solve the equilibrium expression

Assume x is small (100Kb lt Co), so x OH-
NH4 4.2 x 10-4 M and NH3 0.010 - 4.2 x
10-4 0.010 M The approximation is valid !
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Equilibria Involving A Weak Base

• You have 0.010 M NH3. Calc. the pH.
• NH3 H2O ? NH4 OH-
• Kb 1.8 x 10-5
• Step 3. Calculate pH
• OH- 4.2 x 10-4 M
• so pOH - log OH- 3.37
• Because pH pOH 14,
• pH 10.63

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Acid-Base Properties of Salts

• MX H2O ----gt acidic or basic solution?
• Consider NH4Cl
• NH4Cl(aq) ----gt NH4(aq) Cl-(aq)
• (a) Reaction of Cl- with H2O
• Cl- H2O ----gt HCl OH-
• base acid acid base
• Cl- ion is a VERY weak base because its conjugate
  acid is strong.
• Therefore, Cl- ----gt neutral solution

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Acid-Base Properties of Salts

• NH4Cl(aq) ----gt NH4(aq) Cl-(aq)
• (b) Reaction of NH4 with H2O
• NH4 H2O ----gt NH3 H3O
• acid base base acid
• NH4 ion is a moderate acid because its conjugate
  base is weak.
• Therefore, NH4 ----gt acidic solution
• See TABLE 17.4 for a summary of acid-base
  properties of ions.

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Acid-Base Properties of Salts
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Acid-Base Properties of Salts

• Calculate the pH of a 0.10 M solution of Na2CO3.
  
• Na H2O ---gt neutral
• CO32- H2O ? HCO3- OH-
• base acid acid base
• Kb 2.1 x 10-4
• Step 1. Set up ICE table
• CO32- HCO3- OH- initial
• change
• equilib

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Acid-Base Properties of Salts
Calculate the pH of a 0.10 M solution of Na2CO3.
Na H2O ---gt neutral CO32- H2O ?
HCO3- OH- base acid acid base Kb
2.1 x 10-4

• Step 1. Set up ICE table
• CO32- HCO3- OH- initial 0.10 0 0
• change -x x x
• equilib 0.10 - x x x

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Acid-Base Properties of Salts

• Calculate the pH of a 0.10 M solution of Na2CO3.
• Na H2O ---gt neutral
• CO32- H2O ? HCO3- OH-
• base acid acid base
• Kb 2.1 x 10-4

Step 2. Solve the equilibrium expression
Assume 0.10 - x 0.10, because 100Kb lt Co x
HCO3- OH- 0.0046 M
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Acid-Base Properties of Salts

• Calculate the pH of a 0.10 M solution of Na2CO3.
• Na H2O ---gt neutral
• CO32- H2O ? HCO3- OH-
• base acid acid base
• Kb 2.1 x 10-4

Step 3. Calculate the pH OH- 0.0046 M pOH
- log OH- 2.34 pH pOH 14, so pH
11.66, and the solution is ________.
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Lewis Acids Bases

• Lewis acid
• a substance that accepts an electron pair

Lewis base a substance that donates an electron
pair
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Reaction of a Lewis Acid and Lewis Base

• New bond formed using electron pair from the
  Lewis base.
• Coordinate covalent bond
• Notice geometry change on reaction.

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Lewis Acids Bases

• Formation of hydronium ion is also an excellent
  example.

• Electron pair of the new O-H bond originates on
  the Lewis base.

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Lewis Acid/Base Reaction
70

• Other good examples involve metal ions.

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• The combination of metal ions (Lewis acids) with
  Lewis bases such as H2O and NH3
• ------gt COMPLEX IONS

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Reaction of NH3 with Cu2(aq)
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The Lewis Acid-Base Chemistry of Nickel(II)
74

• Ni(H2O)62 6 NH3 ---gt Ni(NH3)62

DMG
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Lewis Acid-Base Interactions in Biology

• The heme group in hemoglobin can interact with O2
  and CO.
• The Fe ion in hemoglobin is a Lewis acid
• O2 and CO can act as Lewis bases

Heme group
76

• Many complex ions containing water undergo
  HYDROLYSIS to give acidic solutions.
• Cu(H2O)42 H2O ---gt Cu(H2O)3(OH) H3O

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• This explains why water solutions of Fe3, Al3,
  Cu2, Pb2, etc. are acidic.

This interaction weakens this bond
Another H2O pulls this H away as H
78
Amphoterism of Al(OH) 3
79

• This explains AMPHOTERIC nature of some metal
  hydroxides.
• Al(OH)3(s) 3 H --gt Al3 3 H2O
• Here Al(OH)3 is a Brønsted base.
• Al(OH)3(s) OH- --gt Al(OH)4-
• Here Al(OH)3 is a Lewis acid.

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• Formation of complex ions explains why you can
  dissolve a ppt. by forming a complex ion.
• AgCl(s) ? Ag Cl- Ksp 1.8 x 10-10
• Ag 2 NH3 --gt Ag(NH3)2 Kform 1.6 x
  107
• -------------------------------------
• AgCl(s) 2 NH3 ? Ag(NH3)2 Cl-
• Knet __________________

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Why?

• Why are some compounds acids?
• Why are some compounds bases?
• Why do acids and bases vary in strength?
• Can we predict variations in acidity or basicity?

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Why is CH3CO2H an Acid?

• 1. The electronegativity of the O atoms causes
  the H attached to O to be highly positive.
• 2. The OH bond is highly polar.
• 3. The H atom of OH is readily attracted to
  polar H2O.

Figure 17.9
83
Acetic acid
Trichloroacetic acid
Ka 1.8 x 10-5
Ka 0.3

• Trichloroacetic acid is a much stronger acid
  owing to the high electronegativity of Cl.
• Cl withdraws electrons from the rest of the
  molecule.
• This makes the OH bond highly polar. The H of
  OH is very positive.

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Basicity of Oxoanions
NO3-
CO32-
PO43-

• These ions are BASES.
• They become more and more basic as the negative
  charge increases.
• As the charge goes up, they interact more
  strongly with polar water molecules.

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What is a polymer?

• Many mers. (Compare MONOMERS).