Chapter 10
Monoprotic Acid and Bases
1
Strong Acids and Bases

Compound that when dissolved in water will fully
dissociate.

This is a factor of our very universal solvent and its’ special
properties. This is called the leveling effect of water.

Our strong acids are

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
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HCl
HBr
HI
HNO3
HClO4
H2SO4 (first ionization)
2
Strong Acids

The acid dissociation equilibrium

HA + H2O = H3O+ + A-

The equilibrium constant values for our strong acids:
Acid
HCl
HBr
HI
HNO3

Ka
7900
630 000
25 000 000 000
25
3
4
5
Strong Acids

The strongest of the common strong acids is
perchloric acid. (HClO4)

This strengths can be shown be going to other
solvents other than water. Acidic acid is a
common choice.
6
Strong Acid and Bases

Our strong bases are

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NaOH
KOH
LiOH
Sr(OH)2
7
Determining pH

Since they completely dissociate we get the
same concentration of H+ as the dissolved
acid or base.



So if a solution is made up to be 1.00x10-2 M HCl
then the pH will be pH = - log [H+] = -log
(1.00x10-2) = 2.?????? (How many sig figs?)
More correctly we should correct for activity so we
should have pH = -logAH+ = -log g[H+] = -log
(0.914)(1.00x10-2) = 2.03905 (How many sig figs?)
But we will not bother to correct unless called on
to do so.
8
Strong Acids

pH of strong base solutions.

pH of a 1.0x10-3 M NaOH solution.



Since [H+][OH-] = Kw

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[Na+] = .0010 and [OH-] = 0.0010
Then pOH is 3.00
pH + pOH = 14.00
Then pH of this solution is 11.00
Kw varies with temperature so you need to account for that
also.
9
Strong Acids


What if we make up a solution that is 1.0x10-5 M
HNO3. The pH would be 5.00.
If we were to dilute this solution by 1000 fold. That
is take 1.00 mL and dilute to 1.00 Liters. What is
the pH then.
10
Strong Acids



It can not be pH 8. That would mean that
we have made up a solution that is basic
from and acid and water???
How would you calculate this?
This is a case for Chapter 9. Systematic
treatment of equilibrium.
11
Strong Acids




What do we know?
CHNO3 = 1.00x10-8
From this we can see that CNO3- = 1.00x10-8
What is the charge balance of this sytem.



[H+] = [NO3-] + [OH-]
We also have our Kw expression.
So [H+] = CNO3 + Kw/[H+]
12
13
Weak Acids/Bases


These are acids and bases that do not fully
dissociate when placed in aqueous solution.
Examples

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Acetic Acid
Benzoic Acid
Hydrofluoric Acid
Ammonia
14
Weak Acids/Bases

Weak acid equilibrium

HA = H+ + A

[ H ][ A ]
Ka 
[ HA]
15
16
Weak Acids and Bases

Weak Base equlibrium (Also called
hydrolysis constant)

B + H2O = BH+ + OH-
[ BH  ][OH  ]
Kb 
[ B]
17
Conjugate Pairs
OH
H
-
O
H
O
O
Formic Acid
Formate Ion
An acid / conjugate base pair
18
Conjugates
H
H
+
N
N
H
Ammonia
Base
H
H
H
H
Ammonium Ion
Conjugate Acid
19
Table G

The Table in the back of the book. Starting
on page AP12 lists all acids and bases as
acids. It will give the proper base name but
show the structure for the conjugate acid
form. When there is more than one acidic
group then the book will indicate which
proton goes with the given pKa.
20
Table G
8- Hydroxyquinoline
4.91 (NH)
+
N
OH
9.81 (OH)
H
21
Buffers



When you add a weak acid and its’ conjugate
then you will get a buffer. Buffers resist the
rapid change in pH when acid or base is
added to the solution.
pH = pKa + log (base/acid)
pH control is important since many processes
are pH dependent.
22
23
24
Buffers

Made two different ways

Mix an acid and its conjugate base



Acetic acid and sodium acetate
Ammonium Chloride and aqueous ammonium
Prepare a solution of an acid or base and
generate the conjugate by addition of
strong acid or base.

Acetic acid and add NaOH
25
Buffers


When adding both forms the
concentration of the buffer will be the
sum of the concentration of both forms
When generating from strong conjugate
then add the moles of acid needed at
the final volume.
26
Preparation




Weight out the number of moles of
buffer needed and dilute to ~80% final
volume.
Place pH electrode into solution
Add strong acid/base until pH is
reached.
Dilute to mark
27
Buffer Capacity


The amount of acid or base that can be
added before the buffer is consumed.
b = dCb / dpH = - dCa / dpH
28
Buffer pH

Ionic Strength

Temperature
29
What about extreme conditions


FHA + FA- = [HA] + [A-]
[Na+] + [H+] = [OH-] + [A-]

[HA] = FHA – [H+] + [OH-]
[A-] = FA- + [H+] – [OH-]

pH = pKa + log {corr A- / corr HA}

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