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Lecture 18 3/1/06
What is a buffer?
pH of a buffer system
What is the pH of a buffer that is 0.12 M lactic acid HC3H5O3 and 0.1 M sodium lactate?
Ka = 1.4 x 10-4
pH using Henderson-Hasselbach Equation
What is the pH of a buffer that is 0.12 M lactic acid HC3H5O3 and 0.1 M sodium lactate?
Ka = 1.4 x 10-4
Addition of strong acid/strong base
Always a 2 step process:Neutralization (100% completion)Equilibrium
A buffer is prepared by mixing 0.15 mol of lactic acid and 0.2 mol of sodium lactate and raised to 1 L with water.Ka = 1.4 x 10-4
a) pH = ?b) pH after adding 0.05 mol HClc) pH after adding 0.1 mol NaOH
So what determines pH of a buffer?
]HA[
]A[logpKpH a
Buffer Capacity
How much acid/base can be neutralized by a buffer with 0.25 M CH3COOH and 0.25 M of CH3COO- in 1 L.
Titration
Titration Curve
Why do you use one?
Equivalence point vs. Endpoint
pH vs. acid/base added
Building a titration curve via calculations
4 Regions (to build one from calculations):1) initial pH2) before equivalence point3) at equivalence point4) after equivalence point
Building a titration curve via calculationsComparison of a titration of a strong acid vs. a weak acid
Region x-axis (mL of acid/base)
0.1 M NaOH
y-axis (pH)
100 mL of 0.1 M HCl
y-axis (pH)
100 mL of 0.1 M HF
Initial pH
Before the equivalence point(1/2 equiv. point)
equivalence point
After the equivalence point
Initial pH (NaOH titrating HCl)
X-axis:
Y-axis:
Building a titration curve via calculationsComparison of a titration of a strong acid vs. a weak acid
Region x-axis (mL of acid/base)
0.1 M NaOH
y-axis (pH)
100 mL of 0.1 M HCl
y-axis (pH)
100 mL of 0.1 M HF
Initial pH 0 1Before the equivalence point(1/2 equiv. point)
equivalence point
After the equivalence point
0
1
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14
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
Equivalence point (NaOH titrating HCl)
X-axis
Y-axis
Building a titration curve via calculationsComparison of a titration of a strong acid vs. a weak acid
Region x-axis (mL of acid/base)
0.1 M NaOH
y-axis (pH)
100 mL of 0.1 M HCl
y-axis (pH)
100 mL of 0.1 M HF
Initial pH 0 1Before the equivalence point(1/2 equiv. point)
equivalence point 100 7
After the equivalence point
0
1
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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
½ Equivalence point (NaOH titrating HCl)
X-axis
Y-axis
Building a titration curve via calculationsComparison of a titration of a strong acid vs. a weak acid
Region x-axis (mL of acid/base)
0.1 M NaOH
y-axis (pH)
100 mL of 0.1 M HCl
y-axis (pH)
100 mL of 0.1 M HF
Initial pH 0 1Before the equivalence point(1/2 equiv. point)
50 1.47
equivalence point 100 7
After the equivalence point
0
1
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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
After Equivalence point (NaOH titrating HCl)
X-axis
Y-axis
Building a titration curve via calculationsComparison of a titration of a strong acid vs. a weak acid
Region x-axis (mL of acid/base)
0.1 M NaOH
y-axis (pH)
100 mL of 0.1 M HCl
y-axis (pH)
100 mL of 0.1 M HF
Initial pH 0 1Before the equivalence point(1/2 equiv. point)
50 1.47
equivalence point 100 7
After the equivalence point
150 12.3
0
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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
0
1
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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
Sulfanilic acid, which is used in making dyes is made by reacting aniline with sulfuric acid. The acid has a pKa value of 3.23. The sodium salt of the acid is quite soluble in water. If you dissolve 1.25 g of the salt in 125 mL of solution, what is the pH of the solution?
