Water Part 2.

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Chapter 2 Water Part 2.

• Ionization of water and pH.
• Strong acids and bases.
• Weak acids and bases.
• The Henderson-Hasselbach Eqn
• Titration curves and buffers.
• 6. pH problems.

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1. Ionization of water.
H OH-
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H OH- 10-14 At neutrality, H
OH- 10-7 M
Taking logs of both sides log H -14 log
OH-
Define pH - log H and pOH -log OH-
Then -pH -14 pOH or pH 14 - pOH
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2. Strong acids and bases dissociate completely
in water.
HCl ? H Cl-
What is the pH of a 10-4 M solution of HCl? H
10-4 M pH -log H log 10-4 4
NaOH ? Na OH- What is the pH of a 10-5M
solution of NaOH? OH- 10-5 M pOH 5 but
pH 14- pOH Therefore, pH 8
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• Weak acids do not dissociate completely.
• Acetic acid (H Ac) H Ac ?? H Ac-
• Ammonia (NH3) NH3 H ?? NH4

Acid hydrogen ion donor Base hydrogen ion
acceptor Conjugate acids and bases. You cant
have one without the other.
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4. The Henderson-Hasselbach Equation
HA ?? H A- H A- KA
HA
or H KA A-
HA
Define pK - log KA Therefore, pH pK
log A-
HA
Base
Acid
Consequences A- HA pH pK A-
10HA pH pK 1 A- 0.1HA
pH pK - 1
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5. Titration curves and buffers. The smallest
change in pH occurs when the pH pK.
Buffering region
Taken from Campbell Farrell 5th. Ed.
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What is the conjugate base in each case?
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Phosphate buffers.
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6. .pH problems.

• What is the pH of a 0.1 M solution of H Ac? pK
  4.7

KA H Ac- x2 . Where x
H Ac- H Ac (0.1M-x)
and H Ac 0.1 M-x
H
Assume that xltlt0.1 M. Then KA 10-4.7 x2

0.1 M
X2 10-4.7 10-1 10-5.7
Log x -2.85 pH 2.85
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• How many ml of 0.1N HCl must be added to 100ml
• of 0.1M Tris base to make a buffer of pH 8.3?
• Tris HCl ?? Tris Cl-
• To solve this problem, you need the
  Henderson-Hasselbach
• Equation and a conservation equation. Also,
  pay attention
• to units.
• pH pK log Tris or 8.3 8.0
  Tris
• Tris
  Tris
• Tris 0.3
• Tris

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Conservation Eqns Triso Tris Tris
and Tris HCl Let HCl x Tris
Then, Tris 0.1 M -x
Tris 0.3 0.1M x or 1.3x 0.1 M
and x 0.077M Tris x
How many equivalents of HCl correspond to
0.077M? Eq (eq.) conc (eq./liter) x liters
0.0077 eq. How many ml of 5N HCl is needed to
get 0.0077 eq.? x (l) 0.0077eq
0.00154 l. 1l 5 eq.
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Useful information, 1 mole/l 1 mmole/ml
1umole/ul.
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• When you do problems,
• Give the equation.
• Substitute in the equation.
• Use units.
• Give the answer with units.
• Correct answers without the set up will receive
  zero credit.
• Wrong answers with the correct set up will get
  partial credit.