1. Exam 1 Stats Average 51.8 ± 22.1 High 92.5 (2 of you) Low 14 A 100-83B
82-64 11 C
63-41 15 D
40-22 8 F
21-0 6

2. Chapter 8 Activity

3. 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”

4. Hydrated radius _ + _ +
water
Chloride
Sodium ion

5. Ionic Atmosphere and Shielding
Low ionic strength
High ionic strength - - + + - - - - + + - - + - + - - + - - + - + - + - - - - - + - - + - - - - - - - + + - _ + + -
Calcium
ion
Sulfate
ion
Sodium
ion
Chloride
ion

6. Activity
All ionic species in any equilibrium expression are more accurately expressed as activities.
A Ca2+ = [Ca2+]gCa2+ or
A + H2O ↔ A- + H3O+
Ka = A A- A H3O+ / [A] = [A-]gA-[H3O+]gH3O+ / [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.

7. 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.

8. Ionic strength gCa2+@m=0.03 = ?
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
= ?
Use extended Debye-Huckle eq or Table 8.1 and extrapolation

9. 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?

10. Example 8-12 Solubility of Hg2Br2 in pure water in 0.00100 M KNO3

11. In pure water Hg2Br2  Hg22+ + 2Br- Ksp = [Hg22+]gHg22+ [Br-]2gBr-2
2[Hg22+] = [Br-] and let [Hg22+] = x
Ionic strength is very low.
gHg22+ and gBr- are close to 1.00
so,
Ksp = 4 [x]3 = 5.6E-23
x = 2.4E-8 M

12. 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

13. in 0.00100 M KNO3 m = ½ ((.001)(+1)2 + (0.001)(-1)2 = 0.001 M
m = = 0.867
m = = 0.964
Ksp = 4 gHg22+ gBr-2 [x]3 = 5.6E-23
x = 2.6E-8M

14. in 0.0100 M KNO3 m = ½ ((.01)(+1)2 + (0.01)(-1)2 = 0.01 M
m = = 0.660
m = = 0.898
Ksp = 4 gHg22+ gBr-2 [x]3 = 5.6E-23
x = 3.0E-8M

15. in 0.100 M KNO3 m = ½ ((0.1)(+1)2 + (0.1)(-1)2 = 0.1 M
m = = 0.335
m = = 0.75
Ksp = 4 gHg22+ gBr-2 [x]3 = 5.6E-23
x = 4.2E-8M

17. pH and ionic strength True definition of pH
pH = -logAH+ = -log {[H+]gH+}
pH of a M HCl solution
Ionic strength, m, = 0.001; gH+ = 0.967
pH = -log(.001*.967) = 3.01
pH of a M HCl solution
Ionic strength, m, = 0.1; gH+ = 0.83
pH = -log(0.1*0.83) = 1.08
pH of a M HCl/0.100 M NaCl solution
pH = -log(0.001*0.83) = 3.08

18. What is the concentration H+ an OH- in a 0.100 M NaCl solution?
Kw = [OH-]gOH-[H+]gH+ = 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

19. EDTA Prob 12-31
A 50 mL sample containing Ni2+ is treated with mL of M EDTA to complex all of the Ni2+. The excess EDTA is back-titrated, requiring 5.00 mL of M Zn2+. What is the concentration of Ni2+ IN THE ORIGINAL SAMPLE?