Buffers and Titrations - Birdville High School
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Transcript Buffers and Titrations - Birdville High School
Buffers and Titrations
Chapter 19
The Common Ion Effect & Buffer Solutions
• ______________- solutions in which the same ion is
produced by two different compounds
• ______________- resist changes in pH when acids or
bases are added to them
– due to common ion effect
Two common kinds of buffer solutions
1 solutions of a ______________plus a soluble
____________________________
2 solutions of a ______________plus a soluble
____________________________
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Weak Acids plus Salts of Weak Acids
For example ~ acetic acid CH3COOH and
sodium acetate NaCH3COO
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Ex. 1) Calculate the concentration of H+ and the pH of a
solution that is 0.15 M in acetic acid and 0.15 M in
sodium acetate. Ka = 1.8 x 10-5
(note: sodium acetate completely dissociates)
R CH3COOH + H2O CH3COO- + H3O+
I. 0.15
0.15
0
C. -x
+x
+x
E. 0.15 – x
0.15 + x
x
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0.15 x 0.15 and 0.15 x 0.15
Making these assumptions gives
0.15 x
1.8 10
0.15
5
5
x 1.8 10 M H
pH 4.74
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Compare the acidity of a pure acetic acid solution and the
buffer we just described.
Notice that [H+] is ___ times greater in pure acetic acid
than in buffer solution.
+
Solution
[H ]
0.15 M CH3COOH
1.6 x 10 M
0.15 M CH3COOH
&
0.15 M NaCH3COO
pH
-3
2.80
-5
4.74
1.8 x 10 M
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Weak Bases plus Salts of Weak Bases
Ex.2) Calculate the concentration of OH- and the pH of the
solution that is 0.15 M in aqueous ammonia, NH3, and 0.30
M in ammonium nitrate, NH4NO3. Kb = 1.8 x 10-5
R NH3 + H2O NH4+ + OHI
0.15
C -x
E 0.15 –x
0.30
+x
0.30 + x
0
+x
x
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Substitute these values into the ionization expression for ammonia
and solve algebraically.
Kb
NH OH
1.8 10
4
5
NH 3
0.30 x x
5
Kb
1.8 10
0.15 x
apply the simplifying assumption
Kb
0.30 x
5
1.8 10
0.15
6
x 9.0 10 M OH
pOH 5.05 and pH 8.95
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Weak Bases plus Salts of Weak Bases
Let’s compare the aqueous ammonia concentration to that
of the buffer described above.
Note, the [OH-] in aqueous ammonia is ____times greater
than in the buffer.
Solution
0.15 M NH3
0.15 M NH3
&
0.15 M NH4NO3
[OH-]
-3
1.6 x 10 M
9.0 x 10-6 M
pH
11.20
8.95
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Henderson-Hasselbach equation
For acids:
For bases:
Remember:
Buffering Action
__________________________________________.
Ex. 3) If 0.020 mole of HCl is added to 1.00 liter of solution
that is 0.100 M in aqueous ammonia and 0.200 M in
ammonium chloride, how much does the pH change? Assume
no volume change due to addition of the gaseous HCl.
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1st ~ Calculate the pH of the original buffer solution
2nd ~ Calculate the concentration of all species after the
addition of HCl.
HCl will react with some of the ammonia
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3rd ~ Now that you have the concentrations of our salt and
base, you can calculate the new pH.
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4th ~ Calculate the change in pH.
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Ex. 4) If 0.020 mole of NaOH is added to 1.00 liter of solution
that is 0.100 M in aqueous ammonia and 0.200 M in
ammonium chloride, how much does the pH change? Assume
no volume change due to addition of the solid NaOH.
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Preparation of Buffer Solutions
Ex. 5) Calculate the concentration of H+ and the pH of the
solution prepared by mixing 200 mL of 0.150 M acetic acid
and 100 mL of 0.100 M sodium hydroxide solutions.
Determine the amounts of acetic acid and sodium hydroxide
(before reaction)
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Preparation of Buffer Solutions
For ______________ situations, it is sometimes
important to prepare a buffer solution of a given pH.
Ex. 6) A) Find the number of moles of solid ammonium
chloride, NH4Cl, that must be used to prepare 1.00 L of
a buffer solution that is 0.10 M in aqueous ammonia, and
that has a pH of 9.15
B) What mass is needed?
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Acid-Base Indicators
____________________________- point at which
chemically equivalent amounts of acid and base have
reacted
______________- point at which chemical indicator
changes color
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Common Acid-Base Indicators
Color in
Color in
Indicator acidic range pH range basic range
purple
0-2
yellow
Methyl
violet
yellow
3.1-4.4
pink
Methyl
orange
blue
4.7-8.2
red
Litmus
red
8.3-10.0
colorless
Phenolphthalein
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Strong Acid/Strong Base
Titration Curves
______________are graphs that show the pH at
various amounts of titrate added. Allows you to find
the ______________.
For Titration curves, Plot ______________of acid
or base added in titration.
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Ex. 7) Consider the titration of 100.0 mL of 0.100
M perchloric acid with 0.100 M potassium
hydroxide. Find the equivalence point of this rxn.
Plot pH vs. mL of KOH added
1:1 mole ratio
Strong Acid/Strong Base
Titration Curves
Before titration starts the pH of the HClO4 solution is _____
Remember that perchloric acid is a strong acid
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After 20.0 mL of 0.100 M KOH has been added the new
pH is _____.
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After 50.0 mL of 0.100 M KOH has been added the pH
is _____.
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After 90.0 mL of 0.100 M KOH has been added the pH
is ____.
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After 100.0 mL of 0.100 M KOH has been added the pH
is ____.
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Strong Acid/Strong Base
Titration Curves
We’ve calculated only a few points on the titration curve. Similar
calculations for the remainder of titration can show clearly the
__________of the titration curve.
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Weak Acid/Strong Base
Titration Curves
Salts of weak acids and strong bases hydrolyze to give basic
solns so the soln is _______at the equivalence point and the
soln is _____________before the ______________point.
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Strong Acid/Weak Base
Titration Curves
Titration curves for Strong Acid/Weak Bases look similar to Strong
Base/Weak Acid but they are inverted. The soln is _______before
the equivalence point and is __________at the equivalence
point.
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Weak Acid/Weak Base
Titration Curves
Titration curves have ______________vertical sections.
Solution is buffered both ______________and ______________the
equivalence point.
_______________________cannot be used. Instead you can measure
the ______________ in order to find the end point.
The math is complex, we will not worry about it in AP Chem.
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Fun Chemistry problem for you
Blood is slightly basic, having a pH of 7.35 to 7.45. What
chemical species causes our blood to be basic? How does our
body regulate the pH of blood?
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