Exam 1 Stats

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Exam 1 Stats

• Average 51.8 22.1
• High 92.5 (2 of you)
• Low 14

A 100-83 4
B 82-64 11
C 63-41 15
D 40-22 8
F 21-0 6
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Chapter 8Activity
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The true nature of ionic species in solution

• Ions are charged molecules
• As a result, they tend to attract polar solvent
  molecules (like water, for instance) and other
  ions
• Hydrated radius
• Ionic atmosphere
• The thickness of the ionic atmosphere is a
  function of the ionic strength of the solution
  the concentration of charge

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Hydrated radius
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water
Chloride
Sodium ion
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Ionic Atmosphere and Shielding
Low ionic strength
High ionic strength
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Calcium ion
Chloride ion
Sulfate ion
Sodium ion
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Activity

• All ionic species in any equilibrium expression
  are more accurately expressed as activities.
• A Ca2 Ca2gCa2
• or
• A H2O ? A- H3O
• Ka A A- A H3O / A A-gA-H3OgH3O / A
• where g is the activity coefficient and is a
  function of the ionic radius of the ion and the
  ionic strength of the solution.

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Activity - continued

• The higher the ionic strength of the solution,
  the larger the smaller the activity coefficient.
• Rationalization At higher ionic strengths the
  ion cloud around any ion is thicker, which
  weakens the attractive forces between the ion and
  its counterpart, inhibiting recombination.

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Ionic strength

• A measure of the concentration of ions in
  solution
• m ½ ? cizi2
• 0.010 M Ca(NO3)2
• m ½ (NO3-(-1)2 Ca2(2)2)
• ½ (0.02)(-1)2 (0.01)(2)2 0.03 M
• gCa2_at_m0.03 ?
• Use extended Debye-Huckle eq or Table 8.1 and
  extrapolation

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Take home message

• At high ionic strengths, solubility increases
  slightly (by a factor of 2-10).
• pH is influenced by ionic strength
• Weak acid and base dissociation is influenced a
  little by ionic strength
• Significant figures?

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Example 8-12

• Solubility of Hg2Br2
• in pure water
• in 0.00100 M KNO3
• in 0.0100 M KNO3
• in 0.100 M KNO3

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In pure water

• Hg2Br2 ? Hg22 2Br-
• Ksp Hg22gHg22 Br-2gBr-2
• 2Hg22 Br- and let Hg22 x
• Ionic strength is very low.
• gHg22 and gBr- are close to 1.00
• so,
• Ksp 4 x3 5.6E-23
• x 2.4E-8 M

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Activity coefficient as a function of ionic
strength Table 8 -1
m 0.001 m 0.01 m 0.1
Hg22 0.867 0.660 0.335
Br- 0.964 0.898 0.75
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in 0.00100 M KNO3

• m ½ ((.001)(1)2 (0.001)(-1)2 0.001 M
• gHg22 _at_ m 0.001 0.867
• gBr-2 _at_ m 0.001 0.964
• Ksp 4 gHg22 gBr-2 x3 5.6E-23
• x 2.6E-8M

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in 0.0100 M KNO3

• m ½ ((.01)(1)2 (0.01)(-1)2 0.01 M
• gHg22 _at_ m 0.001 0.660
• gBr-2 _at_ m 0.001 0.898
• Ksp 4 gHg22 gBr-2 x3 5.6E-23
• x 3.0E-8M

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in 0.100 M KNO3

• m ½ ((0.1)(1)2 (0.1)(-1)2 0.1 M
• gHg22 _at_ m 0.001 0.335
• gBr-2 _at_ m 0.001 0.75
• Ksp 4 gHg22 gBr-2 x3 5.6E-23
• x 4.2E-8M

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pH and ionic strength

• True definition of pH
• pH -logAH -log HgH
• pH of a 0.00100 M HCl solution
• Ionic strength, m, 0.001 gH 0.967
• pH -log(.001.967) 3.01
• pH of a 0.100 M HCl solution
• Ionic strength, m, 0.1 gH 0.83
• pH -log(0.10.83) 1.08
• pH of a 0.00100 M HCl/0.100 M NaCl solution
• Ionic strength, m, 0.1 gH 0.83
• pH -log(0.0010.83) 3.08

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What is the concentration H an OH- in a 0.100 M
NaCl solution?

• Kw OH-gOH-HgH 1.01E-14
• At m 0.100, gOH- 0.76 and gH 0.83
• H2O is the only source of H and OH- so let x
  H OH-
• Kw (0.76)(0.83) x2
• x 1.27E-7 M

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EDTA Prob 12-31

• A 50 mL sample containing Ni2 is treated with
  25.00 mL of 0.0500 M EDTA to complex all of the
  Ni2. The excess EDTA is back-titrated,
  requiring 5.00 mL of 0.0500 M Zn2. What is the
  concentration of Ni2 IN THE ORIGINAL SAMPLE?