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COMMON ION EFFECT
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COMMON ION an ion common with one in a system at equilibrium which places a stress on the equilibrium Common Ion
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Uses of Common Ion Effect
1. control pH of a weak acid or base 2. control formation of a precipitate
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BUFFER Example Non-example
A solution which resists a change in pH when an acid or base is added consists of a weak acid or base and a salt containing a common ion of its conjugate
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LeChatelier’s Principle explain the
How does LeChatelier’s Principle explain the operation of a buffer?
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Example of a buffer system
CH3COOH + HOH CH3COO- + H3O+ NaCH3COO(aq) Na+ + CH3COO-
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Characteristics of a Good Buffer
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1. operates over a narrow pH range (< 1 pH unit)
2. no reactions between buffers in a multiple buffer system 3. range can be extended using more than one buffer
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Henderson-Hasselbalch
Equation
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Maximum buffering will occur when ratio is close to 1, or when
pH = pKa
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1. What is the pH of a 0.20 M acetic acid solution?
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Add 10. 0 mL of 0. 20 M NaOH to 50. 0 mL of the preceding solution
Add 10.0 mL of 0.20 M NaOH to 50.0 mL of the preceding solution. What is the pH?
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Add 5. 0g sodium acetate (MM 82. 05) to 500. mL of the 0
Add 5.0g sodium acetate (MM 82.05) to 500. mL of the 0.20 M acetic acid solution. What is the pH?
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Add 10. 0 mL of 0. 20 M NaOH to 50. 0 mL of the preceding solution
Add 10.0 mL of 0.20 M NaOH to 50.0 mL of the preceding solution. What is the pH?
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2. Calculate the mass of ammonium chloride (MM 43
2. Calculate the mass of ammonium chloride (MM 43.6) needed to buffer 250. mL of 2.0 M ammonia to a pH of 10.
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TITRATION CURVES
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A graphical history of a titration
Titration Curve A graphical history of a titration typically a plot of the pH (dependent variable) and volume titrant (independent variable)
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Uses of Titration Curves
1. determine equivalence point 2. determine number of ionization reactions 3. determine optimum buffer region 4. determine possible indicators
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Shape of Titration Curve
Strong acid - strong base Weak acid - strong base
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Shape of Titration Curve
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Shape of Titration Curve
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Equivalence Point 1. Midpoint between points of inflection
2. Plot of the slope of each point of the curve against volume titrant (DpH/DV vs Vavg)
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Number of Ionization Reactions
CH3COOH - NaOH H2C2O4 - NaOH
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Optimum Buffer Region Area where the concentration of molecules and their conjugate ions are relatively high
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Indicators Need to choose for each titration system
Dependent on pH at equivalence point
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ACID-BASE INDICATORS
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Acid-base indicators are weak Bronsted-Lowry compounds that are different colors in acid and base form.
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Acid-base indicators are all large organic molecules.
HIn <===> H+ + In- Color Color 2
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Phenolphthalein Colorless acid form, HIn
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Phenolphthalein Pink base form, In-
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The color change occurs at a different pH for different indicators.
The pH at which the indicator changes color is dependent on the Ka of the indicator as a weak acid.
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HIn <===> H+ + In-
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Experiments have shown that the minimum amount of change of
HIn <==> In- that can be detected visually is
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Thus, from the Henderson-Hasselbalch equation, one can select an appropriate indicator for a titration based upon the Ka of the indicator and the pH at the equivalence point.
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What is the pH at the equivalence point of a titration of 25
What is the pH at the equivalence point of a titration of 25.0 mL each of 0.10 M HCl and 0.10 M NaOH?
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What is the pH at the equivalence point of a titration of 25
What is the pH at the equivalence point of a titration of 25.0 mL each of 0.10 M CH3COOH and 0.10 M NaOH?
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Phenolphthalein Ka = 1 x 10-9 pH of perceptible color change?
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SOLUBILITY EQUILIBRIA
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Saturated Solution Maximum amount of solute dissolved in a specific volume of solvent at a specific temperature
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Saturated Solution Equilibrium is established between a solid solute and ions from the solute
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Super-Saturated Solution
More than the normal maximum amount of solute is dissolved in a solution.
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Question at a constant temperature, what is the difference in concentration of a saturated solution: (1 mL vs 1 ML solution) (1 mg vs 1 kg solid)
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The concentration of a saturated solution remains the same, no matter how much solid is present, as long as the temperature remains constant.
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The “concentration” of a solid remains the same at a constant temperature.
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solid <===> solution AgCl(s) <===> Ag+ + Cl-
By convention, equations for the formation of saturated solutions are written in the format solid <===> solution AgCl(s) <===> Ag+ + Cl-
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AgCl(s) <===> Ag+ + Cl-
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AgCl(s) <===> Ag+ + Cl-
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3. What is the solubility of silver chloride in water at 25oC?
(Ksp = 1.6 x 10-10)
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4. What is the solubility of lead(II) bromide at 25oC. (Ksp = 4
4. What is the solubility of lead(II) bromide at 25oC? (Ksp = 4.6 x 10-6)
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6. What mass of nickel is dissolved in 100
6. What mass of nickel is dissolved in 100. mL of saturated nickel(II) hydroxide? (Ksp = 1.6 x 10-16) What is the pH of this solution?
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Which is more soluble? Ag2CO3 [Ksp = 8.5 x 10-13] or CaCO3 [Ksp = 3.4 x 10-9]
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SOLUBILITY ---- ACIDITY PRECIPITATION
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8. If 0. 581 gram of magnesium hydroxide (MM 58. 3) is added to 1
8. If gram of magnesium hydroxide (MM 58.3) is added to 1.00L of water, will it all dissolve? (Ksp = 8.9 x 10-12) Below what pH would the solution be buffered so that it does all dissolve?
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9. Calculate the concentration of NH4+ from ammonium chloride required to prevent the precipitation of Ca(OH)2 in a liter of solution that contains 0.10 mole of ammonia and 0.10 mole of calcium ion.
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10. If 50. mL of 0. 012M barium chloride are mixed with 25 mL of 1
10. If 50. mL of 0.012M barium chloride are mixed with 25 mL of 1.0 x 10-6M sulfuric acid, will a precipitate form? HINT: use the concentration quotient “Q” as we used it before
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You have a aqueous solution of Zn2+ and Pb2+ both 0. 0010 M
You have a aqueous solution of Zn2+ and Pb2+ both M. Both form insoluble sulfides. Approximately what pH will allow maximum precipitation of one ion and leave the other in solution? [Ksp ZnS = 2.5 x 10-22] [Ksp PbS = 7 x 10-29]
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SOLUBILITY ---- COMMON IONS COMPLEX IONS
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12. Calculate the molar solubility of silver thiocyanate, AgSCN, in pure water and in 0.010M NaSCN.
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A charged species consisting of a metal ion surrounded by ligands
Complex Ion A charged species consisting of a metal ion surrounded by ligands
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LIGAND An ion or molecule, acting as a Lewis base, attached to the central metal ion using the d-orbitals of the metal
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The number of ligands attached to the central metal ion.
Coordination Number The number of ligands attached to the central metal ion. 2, 4, or 6 are most common CN
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Metal ions add ligands one step at a time.
Ag+ + NH3 <==> Ag(NH3)+ Kf1 = 2.1 x 103 Ag(NH3)+ + NH3 <==> Ag(NH3)2+ Kf2 = 8.2 x 103 where Kf = formation constant
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You need to familiarize yourself with “typical” complex ions, Appendix K
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Note that a formation constant reflects the stability of the complex.
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13. Calculate the equilibrium constant for
AgI(s) + 2NH3(aq) <===> [Ag(NH3)2]+(aq) + I-(aq)
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14. Will 5.0 mL of 2.5 M NH3 dissolve 0.0001 mole AgCl?
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15. A solution is prepared by adding 0. 10 mole Ni(NH3)6Cl2 to 0
15. A solution is prepared by adding 0.10 mole Ni(NH3)6Cl2 to 0.50 L of 3.0 M NH3. Calculate the [Ni(NH3)62+] and [Ni2+] in the solution.
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