UNIT 13
Acids, Bases and Salts
Arrhenius definition of acids and bases:
Acid: a chemical compound that increases the concentration of hydrogen ions (H+) in
aqueous solution
HCl (aq) → H+ (aq) + Cl- (aq)
Base: a substance that increases the concentration of hydroxide ions (OH -) in
aqueous solution
NaOH (aq) → Na+ (aq) + OH- (aq)
Properties of Acids
Properties of Bases
Acids:
HCl + H2O ↔ H3O+ + Cl-
Bases:
NaOH + H2O → Na+ + OH-
• taste sour
• corrosive
• electrolytes (strong)
• react with indicators to change color
feel slippery
taste bitter
corrosive
electrolytes (strong)
react with indicators to change color
Common acids
Four most important industrial acids:
1 - sulfuric acid
formula: H2SO4
uses: car batteries
Common bases
1 – ammonia - most widely used
NH3 + H2O → NH4+ + OH- → NH4OH
formula: NH3
uses: clenaers, rayon/nylon
production
2 - phosphoric acid
formula: H3PO4
uses: detergents
2 - calcium hydroxide - “caustic lime”
formula: Ca(OH)2
uses: mortar, plaster
3 - nitric acid
formula: HNO3
uses: fertilizer, explosives (TNT)
3 – sodium hydroxide – “lye”
formula: NaOH
uses: soap, drain cleaner
4 – hydrochloric acid (muriatic acid)
formula: HCl
uses: cleaner, gastric juices
1
Brønsted- Lowry definition of acids and bases:
Acid: proton (H+) donor
Base: proton (H+) acceptor
Label the parts of the general reaction for a Brønsted- Lowry acid and water:
HA (aq) + H2O (l) ↔ H3O+ (aq) + A- (aq)
acid
base
conj acid
conj base
Conjugate Base: Everything that remains of the acid molecule after a proton (H +) is lost.
Conjugate Acid: The base with the transferred proton (H+).
Conjugate Acid/Base Pair: two substances related to each other by the donating and
accepting of a single proton.
Example: Finish each equation and identify each member of the conjugate acid/base pair.
a.
H2SO4
acid
(aq)
+ H2O (l) ↔ HSO4- + H3O+
base
conj base conj acid
b. CO32- (aq) + H2O (l) ↔ HCO3- + OHbase
acid
conj acid conj base
The hydronium ion, H3O+, forms when water behaves as a base.
(The two unshared pair of electron on the O bond covalently with the H+)
2
Example: Write the conjugate base for each of the following:
a.
HClO4 → ClO4-
b. H3PO4 → H2PO4c.
CH3NH3+ → CH3NH2
Diprotic Acid: an acid that can provide two protons
H2SO4
(aq)
→ HSO4-
HSO4-(aq) → SO42Water as an Acid and a Base
Amphoteric: a substance that can behave as either an acid or a base
(water is the most common)
Ionization of water:
H2O (l) + H2O (l) ↔ H3O+(aq) + OH-(aq)
In the shorthand form:
H2O (l) ↔ H+(aq) + OH-(aq)
There are three possible situations:
1. A neutral solution where [H+] = [OH-]
2. An acidic solution where [H+] > [OH-]
3. A basic solution where [H+] < [OH-]
Ion-product constant: Kw refers to the ionization of water
Kw = [H+] [OH-]
3
At 25 ºC,
Kw
=
[H+] [OH-]
[1.0 x 10-7] [1.0 x 10-7]
=
=
1.0 x 10-14
If [H+] increases, [OH-] decreases, two the product of the two is still 1.0 x 10 -14
Example:
a. Calculate [H+] or [OH-] as required for each of the following at 25 ºC.
b. For each solution state whether it is neutral, acidic or basic.
1) 1.0 x 10-5 M OH-
2) 1.0 x 10-7 M OH-
+
3) 10.0 M H+
1.0 x 10-9 M H
1.0 x 10-7 M H
1.0 x 10-15 M OHbasic
neutral
acidic
+
pH scale – because the [H ] in an aqueous solution is typically small, logarithms are used to
express solution acidity.
Mental math: calculating pH:
substance
+
concentration (M)
pH
[H3O+]
1 x 10-9
9
acid, base or
neutral?
base
[H3O+]
1 x 10-6
6
acid
[H3O+]
1 x 10-2
2
acid
[H3O+]
1 x 10-11
11
base
[H3O+]
1 x 10-7
7
neutral
[OH-]
1 x 10-9
(pOH = 9)
5
acid
[OH-]
1 x 10-6
(pOH = 6)
8
base
[OH-]
1 x 10-2
(pOH = 2)
12
base
[OH-]
1 x 10-11
(pOH = 11)
3
acid
[OH-]
1 x 10-7
(pOH = 7)
7
neutral
pH = -log [H+]
pOH = -log [OH-]
4
pH = -log [H+]
pOH = -log [OH-]
graphing calculator steps:
1. Press the +/- key
2. Press the log key
3. Enter the [H+]
Significant figure rule for pH: the number of places to the right of the decimal for a log
must be equal to the number of significant figures in the original number.
[H+] = 10-pH
[OH-] = 10-pOH
graphing calculator steps:
1. Press 2nd function, then log
2. Press the +/- key
3. Enter the pH
pH + pOH = 14
Since Kw = 1.0 x 10-14,
Fill in the missing information in the table below:
pH
[H3O+] or [H+]
pOH
[OH-]
Acid or Base?
3.16
6.92 x 10-4
10.84
1.45 x 10-11
Acid
.979
1.05 10-1 M
13.0
9.52 x 10-14
Acid
10.28
5.25 x 10-11
3.72
1.91 x 10-4
Base
7.88
1.30 x 10-8
6.12
7.53 x 10-7 M
Base
7.95
1.12 x 10-8
6.05
8.91 x 10-7
Base
9.32
4.72 x 10-10 M
4.67
2.12 x 10-5
Base
2.67
2.14 x 10-3
11.33
4.68 x 10-12
Acid
12.0
1.02 x 10-12
2.01
9.77 x 10-3 M
Base
10.68
2.09 x 10-11
3.32
4.79 x 10-4
Base
7.03
9.23 x 10-8 M
6.97
1.08 x 10-7
Base
5
Neutralization
Reaction of an acid and a base will always produce water and “a salt,” as shown below
acid
HCl
HBr
+
base
NaOH
→
water
HOH
+
salt
NaCl
+
KOH
→
HOH
+
KBr
When equal amounts of an acid and a base react, there is an exact neutralization with no
remaining H+ or OH-. We can use the equation below to determine how much of an acid or a
base is needed to completely neutralize an acid or a base.
Macid Vacid = Mbase Vbase
Example: What volume of 0.30 M HCl solution is needed to completely react with 1.2 L of
0.85 M NaOH?
Macid Vacid = Mbase Vbase
(0.30) V = (0.85) (1.2)
V = 3.4 L
Neutralization with multiprotic acids
 H2SO4 is a diprotic (2 protons) acid. If you have 4.0 M H 2SO4 solution, what would
be the [H+] in the acidic solution?

H3PO4 is a triprotic (3 protons) acid. If you have 2.0 M H3PO4 solution, what would
be the [H+] in the acidic solution?
6
Example: What volume of a 0.075 M KOH solution is required to react with 0.135 L of 0.45 M
H3PO4?
Macid Vacid = Mbase Vbase
(0.45) (0.135) = (0.075) V
V = 0.81 L
Titration
Experimentally, we will find the amount of acid in a solution by determining how many moles
of base are required to neutralize it.
Use the neutralization formula: Ma Va = Mb Vb
Example: The total acidity in water samples can be determined by titration with a standard
sodium hydroxide solution. What is the total concentration of hydrogen ion, H +, present in a
water sample of 100. mL if the sample requires 7.2 mL of 2.5 x 10 -3 M NaOH to be
neutralized?
Macid Vacid = Mbase Vbase
M (100) = (2.5 x 10-3) (7.2)
M = 1.8 x 10-4 M
7
Indicators
 The most common method to get an idea about the pH of solution is to use an acid
base indicator
 Indicator: organic molecule that has a different color at different pHs
 Litmus paper: red below 4.5 and blue above 8.2.
Blue Litmus
Red Litmus
Acid
turn red
stay red
Base
stay blue
turn blue
base = blue


Other commercial pH papers are able to give colors for every main pH unit.
Universal indicator is a mixture of indicators that is able to provide a full range of
colors for the pH scale
pH curve
 During an acid-base titration, the pH changes in a characteristic way. A pH curve is
found by plotting the pH of the solution being titrated against the volume of
solution added.
 The equivalence point is the point at which equivalent molar amounts of acid [H +]
and base [OH-] have been mixed. When a strong bases is mixed with a strong acid,
the equivalence point is right around pH = 7.
 An indicator (phenolphthalein) is often used in a titration to visually show when the
equivalence point is reached.
 Choose an indicator that changes color near the equivalence point
8
Examples:
1. At what pH are the equivalence points in the pH curves shown above? 7
2. From the list provided, which is the best indicator for giving a color change at the
equivalence point? phenolphthalein, phenol red, bromothymol blue
Strength of acids and bases:
Strong Acid: acid that ionizes almost completely in solution (electrolytes)
HCl(g) + H2O(l) → H3O+(aq) + Cl-
(aq)
Weak Acid: acid that partially ionizes in solution (weak -non electrolytes)
CH3COOH(l) + H2O(l) ↔ H3O+(aq) + Cl-
(aq)
Strong Base: dissociates completely in solution (alkaline)
NaOH + H2O(l) → Na+(aq) + OH-(aq)
Weak Base: partly dissociates in solution
NH3(g) + H2O(l) ↔ NH4+(aq) + OH-
(aq)
9
Strength
Strong
Weak
Acid
HCl, H2SO4, HNO3, HClO4,
HBr, HI
CH3COOH, H2CO3, H3BO3
Chemistry
Acids and Bases Worksheet
Base
NaOH, KOH, LiOH,
Ca(OH)2, Ba(OH)2
NH3, Al(OH)3, Fe(OH)3
Name:________________________
Date:__________________
Water is amphoteric. What does this mean? act as acid or base
Use this equation for questions 1-4:
1.
HNO3
+
H2 O
→
H3O+
NO3-
+
The acid is: HNO3
2. The base is: HOH
3. The conjugate acid is: H3O+
4. The conjugate base is: NO3Use this equation for questions 5-8:
H2PO4-
+
H 2O
→
H3PO4
+
OH-
5. The acid is: HOH
6. The base is: H2PO47. The conjugate acid is: H3PO4
8. The conjugate base is: OH-
9. In the reaction below, label the acid, base, conjugate acid and conjugate base:
HCl + H2O → H3O+ + ClA
B
CA
CB
10. In the reaction below, label the acid, base, conjugate acid and conjugate base:
NH3 + H2O →
B
A
NH4+ + OHCA
CB
10
11. Name 3 properties of acids: taste sour, corrosive, electrolytes (strong), react with
indicators to produce color change
12. Name 3 properties of bases: feel slippery, taste bitter, corrosive, electrolytes
(strong), react with indicators to produce color change
13. What is the equation for expressing pH? pOH?
pH = -log [H+]
pOH = -log [OH-]
14. What is the pH of 0.75 M hydrochloric acid solution?
pH = 0.125
15. What is the pH of 0.001 M barium hydroxide solution?
pH = 11.30
16. Find the pH of a strong base that has a [OH-] of 1.0 x 10-9 M.
pH = 5
17. What is the pOH if the pH is 12.05?
pH = 1.95
18. A solution has a pH of 8.5. What is the pOH?
pOH = 5.5
19. Describe strong acids and strong bases.
Complete dissociation (electrolytes)
11
20. Determine the conjugate acid for each: bases accept H+
H2O H3O+
h.
F- HF
b. HCO3- H2CO3
i.
SO42- HSO4-
OH- H2O
j.
PO43- HPO42-
d. H2PO4- H3PO4
k.
Cl- HCl
a.
c.
e.
ClO4- HClO4
l.
CH3COO - CH3COOH
f.
SH- H2S
m.
CN- HCN
g.
HSO4- H2SO4
n.
NH3 NH4+
21. Determine the conjugate base for each: acids donate H+
H2O OH-
h.
HF F-
b. HCO3- CO32-
i.
HSO4- SO42-
OH- O2-
j.
HPO42- PO43-
k.
HCl Cl-
a.
c.
d. H3PO4 H2PO4e.
HBrO2 BrO2-
l.
CH3COO H CH3COO-
f.
H2S HS-
m.
HOCN OCN-
g.
HSO4- SO42-
n.
NH3 NH2-
12
22. Given the following reactions. Label the acid and base and draw an arrow between
the conjugates. You may remove spectator ions and simply write the net ionic
reaction.
a. HCO3-
+
H 2O
acid
b. HCl
base
+
H 2O
acid
+
acid
acid
H3O+
→
NaOH-
CB
NaH2PO4base
Cl-
+
CA
→
CB
NaCH3CO2- + H2O
base
+
CO32-
+
CA
base
c. CH3CO2H
d. NH4+Cl
H3O+
→
CB
→
NH3
CB
CA
+
H3PO4
+
NaCl
CA
13