Chapter 6 Problems

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Chapter 6 Problems

• 6-29, 6-31, 6-39, 6.41, 6-42, 6-48,

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Outline

• Equilibrium of Acids and Bases
• Bronsted-Lowry Acids/Bases
• Define strong
• Define weak
• pH of pure water at 25oC
• Define Ka and Kb
• Relationship b/w Ka and Kb
• Chapter 8 Activity
• Relationship with K

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Acids and Bases Equilibrium

• Section 6-7

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Strong Bronsted-Lowry Acid

• A strong Bronsted-Lowry Acid is one that donates
  all of its acidic protons to water molecules in
  aqueous solution. (Water is base electron donor
  or the proton acceptor).

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Strong Bronsted-Lowry Base

• Accepts protons from water molecules to form an
  amount of hydroxide ion, OH-, equivalent to the
  amount of base added.

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Question

• Can you think of a salt that when dissolved in
  water is not an acid nor a base?

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Weak Bronsted-Lowry acid

• One that DOES not donate all of its acidic
  protons to water molecules in aqueous solution.
• Example?

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Weak Bronsted-Lowry base

• Does NOT accept an amount of protons equivalent
  to the amount of base added, so the hydroxide ion
  in a weak base solution is not equivalent to the
  concentration of base added.
• example

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Common Classes of Weak Acids and Bases

• Weak Acids
• Weak Bases

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Equilibrium and Water

• Question Calculate the Concentration of H and
  OH- in Pure water at 250C.

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EXAMPLE Calculate the Concentration of H and
OH- in Pure water at 250C.

• H2O H OH-

Kw
KW
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EXAMPLE Calculate the Concentration of H and
OH- in Pure water at 250C.

• H2O H OH-

Kw
KW
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Example

• What is the concentration of OH- in a solution of
  water that is 1.0 x 10-3 M in H (_at_ 25 oC)?

From now on, assume the temperature to be 25oC
unless otherwise stated.
Kw HOH-
1.0 x 10-14 1 x 10-3OH-
1.0 x 10-11 OH-
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pH

• -3 -----gt 16
• pH pOH - log Kw pKw 14.00

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Weak Acids and Bases
Ka

• HA H A-

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Weak Acids and Bases
Kb

• B H2O BH OH-

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Relation Between Ka and Kb
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Relation between Ka and Kb

• Consider Ammonia and its conjugate acid.

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Example

• The Ka for acetic acid is 1.75 x 10-5. Find Kb
  for its conjugate base.

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Example

• Calculate the hydroxide ion concentration in a
  0.0100 M sodium hypochlorite solution.
• OCl- H2O ? HOCl OH-

The acid dissociation constant 3.0 x 10-8
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1st Insurance Problem

• Challenge on page 120

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Chapter 8

• Activity

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Write out the equilibrium constant for the
following expression

• Fe3 SCN- D Fe(SCN)2

Q What happens to K when we add, say KNO3 ?
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Keq
K decreases when an inert salt is added!!! Why?
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8-1 Effect of Ionic Strength on Solubility of
Salts

• Consider a saturated solution of Hg2(IO3)2 in
  pure water. Calculate the concentration of
  mercurous ions.
• Hg2(IO3)2(s) D Hg22 2IO3- Ksp1.3x10-18

some - - -x x 2x some-x x 2x
I C E
A seemingly strange effect is observed when a
salt such as KNO3 is added. As more KNO3 is added
to the solution, more solid dissolves until
Hg22 increases to 1.0 x 10-6 M. Why?
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Increased solubility

• Why?
• LeChateliers Principle?
• Complex Ion?
• ?

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The potassium hydrogen tartrate example
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Alright, what do we mean by Ionic strength?

• Ionic strength is dependent on the number of ions
  in solution and their charge.
• Ionic strength (m) ½ (c1z12 c2z22 )

Or Ionic strength (m) ½ S cizi2
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Examples

• Calculate the ionic strength of (a) 0.1 M
  solution of KNO3 and (b) a 0.1 M solution of
  Na2SO4 (c) a mixture containing 0.1 M KNO3 and
  0.1 M Na2SO4.

(m) ½ (c1z12 c2z22 )
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Alright, thats great but how does it affect the
equilibrium constant?

• Activity Ac Cgc
• AND

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Relationship between activity and ionic strength
Debye-Huckel Equation
m ionic strength of solution g activity
coefficient Z Charge on the species x a
effective diameter of ion (nm)
2 comments

1. What happens to g when m approaches zero?
2. Most singly charged ions have an effective radius
   of about 0.3 nm

Anyway we generally dont need to calculate g
can get it from a table
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Activity coefficients are related to the hydrated
radius of atoms in molecules
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Relationship between m and g
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Back to our original problem

• Consider a saturated solution of Hg2(IO3)2 in
  pure water. Calculate the concentration of
  mercurous ions.
• Hg2(IO3)2(s) D Hg22 2IO3- Ksp1.3x10-18

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Back to our original problem

• Consider a saturated solution of Hg2(IO3)2 in
  pure water. Calculate the concentration of
  mercurous ions.
• Hg2(IO3)2(s) D Hg22 2IO3- Ksp1.3x10-18

In 0.1 M KNO3 - how much Hg22 will be dissolved?
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Back to our original problem

• Consider a saturated solution of Hg2(IO3)2 in
  pure water. Calculate the concentration of
  mercurous ions.
• Hg2(IO3)2(s) D Hg22 2IO3- Ksp1.3x10-18

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