LaurenHill Chemistry 534
17.
An Introduction to Acids and Bases
Acids
Bases
Operational Definitions: If you recall, operational
definitions are based on what you can actually observe in
the lab.
Operational Definitions: If you recall, operational
definitions are based on what you can actually observe in
the lab.

Acids taste sour.

Bases taste bitter.

They conduct electricity.

They conduct electricity.

They destroy the properties of bases.

They destroy the properties of acids.

They turn blue litmus red.

They turn red litmus blue.

They release H2 gas when added to some metals.

They feel slippery.

They turn fats into soaps.
Conceptual Definitions: The Arrhenius definition of an
acid: a substance that releases H+1.
Example: HCl(aq)
 H+1(aq) + Cl-1(aq)
A Brönsted-Lowry acid donates H+ ions.
HCl(aq) + H2O(l)  H3O +1(aq) + Cl-1(aq)
Conceptual Definitions: The Arrhenius definition of a
base: a substance that releases OH-1.
Example: NaOH(aq)
 Na+1(aq) + OH-1(aq).
A Brönsted-Lowry base accepts H+ ions.(notice the OHis being released by water once it “coughs up” the H+ to
the base.
NH3aq) + H2O(l)  NH4 +1(aq) + OH-1(aq)
Everyday substances that are acidic (pH <7) include fruits
(contain citric and/or malic and/or tartaric acids) soda
drinks (carbonic acid and/or phosphoric acid), toilet bowl
cleaner (HCl), muriatic acid brick cleaner (HCl).
Everyday substances that are alKAline (basic) (pH >7)
include baking soda (NaHCO3), soap, milk of magnesia
(Mg(OH)2),ashes (contain KOH) and household ammonia
(NH4OH).
Examples:
1.
2.
a.
Write an ionic equation for the dissociation of HI(aq).
b.
List some physical properties that you expect this substance to have.
a.
Show how X-2 can act as a base.
b.
Show how NH4+ can act as an acid.
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IV. Chemical Equilbrium
3.
Identify two Bronsted Lowry acids in the following:
NH4+(aq) + HSO4-(aq)
18.
Kw, pH and pOH
A.
Kw
NH3 (aq) + H2SO4 (aq)
A small amount of water constantly dissociates into H+1 and OH-1 ions. Of course these
reassociate into water at the same rate to create the following equilibrium:
H2O(l) = H+1(aq) + OH-1(aq).
Kw = [H+1] [OH-1]
Notice we give water's equilibrium constant a special name: " Kw".
Experimentally Kw = 1.0 X 10-14 at 25 oC, so you can see from the small value
for K that there are very few H+1 and OH-1 ions in water.
B.
pH
In an aqueous acidic solution there will be more H+1 than OH-1, but the
latter will exist nevertheless because of water's contribution. Similarly a base
-
+
will produce OH but there will still be a small amount of H from water.
This is why we introduce the pH scale, which can be used to assign
Soren Sorenson 1868-1939:
In chemistry "p" is an abbreviation
for -log. It was introduced by this
Danish biochemist, in 1909— p is
the initial letter of the German word
potenz = power.
a value to any aqueous solution, regardless of whether it's an acid or base.
pH = - log [H+1] or rearranging it mathematically, [H+1] =10-pH
Example 1
If [H+1] = 1.0 X 10-2 M in your stomach, what is its pH?
Solution
pH = - log [H+1]
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LaurenHill Chemistry 534
Example 2
If the pH of tomato juice = 3.5, what is its [H+1]?
Solution
[H+1] =10-pH
Example 3
Why Is Neutral Stuff at a pH = 7?
In pure water, or in any neutral solution, [H+1] = [OH-1], so by substituting, the expression
Kw = [H+1] [OH-1] becomes..........
C.
pOH
pOH = -log[OH-]
[OH-] = 10-pOH
Example 4
How are the sum of pH and pOH related?
[H+1] [OH-1] = 1 X 10-14.
Log both sides:
log ([H+1] [OH-1]) = -14. Remember from math : log AB = log A + log B and continue:
Example 5:
If an aqueous sodium hydroxide solution has [OH-1] = 0.010 M, what its
pOH? Its pH?
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IV. Chemical Equilbrium
Example 6: Complete the following table:
[H+1] mol/L
A
B
[OH-1] mol./L
pH
Neutral, acidic,
or basic
1.0 x 10-3
2.0 X 10-4
3.0
C
D
pOH
6.0
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LaurenHill Chemistry 534
Exercises
1.
Complete the table below.
[H+1] mol/L
A
1.0 X10-7
B
1.5 X 10-3
C
3.0 X 10-10
D
0.10
[OH-1] mol/L
E
2.0 x 10-7
F
4.0 x 10-2
G
1.5 x 10-9
H
0.50
pH
I
Stomach acid
(2.0)
J
Milk (6.77)
K
Household
cleaner (12.0)
L
pOH
Neutral,
acidic, or
basic
1.8
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IV. Chemical Equilbrium
[H+1] mol/L
[OH-1] mol./L
pH
Neutral, acidic,
or basic
pOH
M
Baking soda
(5.74)
N
Lemon juice
(11.5)
2.
Order the solutions from #2 (A to F only) from the strongest to the weakest acid.
Explain which values you used and why.
3.
In an aqueous solution, the H+1 (aq) concentration is 2.8 x 10-4 M. What is the
concentration of OH-1 (aq)?
4.
In an aqueous solution in which the OH-1 (aq) concentration is 1.0 x 10-7 M, what is
the concentration of H+1 (aq)?
5.
The pH of a sample of human blood was measured to be 7.41 at 25 0C. Calculate
pOH, [H+1]aq, and [OH-1]aq for the sample.
6.
Identify 2 Brönsted-Lowry bases and two acids from each of the following:
a.
HCO3- (aq)
+
NH3(aq)
CO3-2(aq)
+
NH4+(aq)
b.
HSO4- (aq)
+
H2PO4- (aq)
H3PO4
+
SO42- (aq)
7.
Show how CH3COOH can act as an acid in water.
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LaurenHill Chemistry 534
19.
KA and KB:
Dissociation Constants for Acids and Bases
A.
KA = ionization (dissociation) constant for an acid
A weak acid is one with a reasonably small K, which implies that only a small
percent of the acid is actually dissociating into H+ and a negative ion.
For HA(aq)
H+(aq) + A -1(aq)
In all the examples we will see there will only be one H+1 coming out of the acid at any
one time, so the ratio of [HA]dissociating into [H+1] will be 1:1, and the ratio of [H+1] to
[A -1] will be 1:1.
The above expression then becomes:
initial
As an alternative to learning yet another formula, see this as another version of previous
equilibrium problems, where you assume that the initial concentration of H+ and its
accompanying ions is zero.
HX(aq)
H+(aq) + X-(aq)
HX(aq)
Initial amount (moles/L)
H+(aq)
0,always
X-(aq)
0, always
Reacting/forming(moles/L)
Equilibrium
amount(moles/L)
KA is an indicator of how strong an acid is compared to a different acid. If KA is very
large there is 1 mole of H+ for every initial mole of HX. So a 0.10 M solution will have a
pH of 1. If KA is low, the acid is said to be weak and an initial concentration identical to
the above will lead to a higher pH.
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IV. Chemical Equilbrium
If we add lots of water to an acid to raise its pH, the acid is still considered to be a strong
type even though the specific sample has very little H+. It is still doing the most with
what it was given because of its strength.
Example 1:
If KA for the acid HX is 1.00 X 10-5, what is the pH of an acid with an
initial concentration of 0.100 M?
H+1(aq) + X-1(aq).
HX(aq)
H+(aq)
HX(aq)
X-(aq)
Initial amount (moles/L)
Reacting/forming(moles/L)
Equilibrium
amount(moles/L)
Example 2
A 0.100 mol/L solution of HCH2CO2H is partially ionized. At equilibrium
[H+1] = 1.34 X 10-3.
Calculate the equilibrium constant of acetic acid, given:
HCH2CO2H (aq)
H+1(aq) + CH3CO2 -1(aq).
HCH2CO2H (aq)
H+1(aq)
CH3CO2 -1(aq)
Initial
Reacting/forming
Equilibrium
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LaurenHill Chemistry 534
Example 3
Which is the stronger acid?
HX
HA
Initial concentration
0.010
1.0 X 10-5M
pH
4.0
5.1
Note the wording in the following:
Acid
HCl
KA
very large
pH
1.0
4.0
H(CH3COO)
1.8 X 10-5
5.1
2.4
A
concentrated
solution of a
strong
acid(strong
because of
high Ka)
A diluted
solution of a
strong acid
Diluted
solution of a
weak acid
More
concentrated
solution of a
weak acid
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IV. Chemical Equilbrium
B.
KB
KB = dissociation constant for a base. Handle all Kb problems as if they were KA
problems, except that they relate to bases, not acids.
Example 1
If the equilibrium concentration of XOH is 0.10 M, and the pOH = 3.0,
what is the KB of XOH?
(First write an equation)
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LaurenHill Chemistry 534
Example 2
A chemist dissolved 0.3 moles of cocaine
(C17H21NO4) in a liter of solution and its
pOH was 2.35.
First write an equation to show how it acts
as a Bronsted-Lowry base in water, and
then find Kb.
Cocaine is a topical anesthetic that was
used in eye and throat surgery in the 19th
and early 20th centuries. But in the 1900’s
it gained popularity as a recreational drug.
Its
highly
addictive
psychological
properties often destroy people both
financially and emotionally.
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IV. Chemical Equilbrium
OH
Example 3
O
A policeman brings you a white powder that might
be morphine sulphate (molar mass 383.1 g/mole),
a strong pain killer that is physiologically addictive.
Its pKB is 6.10. If the pH of a 100.0 ml sample is
10.4, how many grams of the white powder should
appear upon evaporating the water, if the sample is
truly morphine sulfate?
S
O
HO
HO
HO
C17H21NO7S(aq) + H2O(l) = C17H21NHO7S
+
(aq)
-
+ OH
aq)
N
O
H
morphine sulfate
More on morphine? Read:
http://www.emsb.qc.ca/laurenhill/science/morphine.html
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LaurenHill Chemistry 534
Exercises
1.
Formic acid (HCO2H) is secreted by ants. At equilibrium, pH is 2.7 and
KA = 1.8 x 10-4. What is the concentration of formic acid at equilibrium?
HCO2H (aq)
H+1 (aq) + CO2H-1 (aq)
2.
The hypoclorite ion (OCl-1) is often found in household bleaches and
disinfectants. It is related to HOCl, the active ingredient that forms when
swimming pool water is treated with chlorine-compounds. Weakly acidic,
hypochlorous acid has a KA of3.5 X 10-8). Calculate the pH of an aqueous
solution of hypochlorious acid when concentration of HOCl (aq) at
equilibrium is 0.100 M.
3.
At 25 0C, HF(aq) dissociates and reaches an equilibrium. The KA of the solution is
4.7 x 10-4. What is the concentration of H+1(aq) in the solution when initial
concentration of HF(aq) is 2.0 M?
4.
A 0.011 M solution of hydrocyanic acid, HCN dissociates in solution. The
concentration of H+1 (aq) at equilibrium is 5.8 x 10-3 M. What is the KA of this
solution?
5.
In 80.0 mL solution, 0.25 mol of boric acid, H3BO3 dissociates into 3.4 x 10-4 mol
of H2BO3-1 (aq). What is the KA and pH of this solution?
6.
The acid dissociation constant of NH4+1 is 5.6 x 10-10. When a 0.100 M solution of
NH4+1 dissociates and reaches equilibrium, what is the concentration of H+1(aq) and
pOH of the solution?
NH4+1(aq)
NH3 (aq) +
H+1(aq)
7.
The initial concentration NH4OH(aq) is 0.12 M. The pH of the solution is 10.0.
What is the base dissociation constant, Kb of NH4OH?
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IV. Chemical Equilbrium
8.
Codeine is a derivative of morphine that is used as an analgesic, narcotic, or
antitussive. It was once commonly used in cough syrups but it is now
available only by prescription because of its addictive properties. The
molecular mass of codeine is 300.0 g/mol, and the pKb is 6.05. Calculate
the pH of a 10.0 mL solution containing 5.0 mg codeine. (Note that pKb =
-log Kb and 1 mg = 10-3 g)
Codeine (aq)
9.
CodeineH+1 (aq)
+
OH-1 (aq)
Use the information in the following table to determine which acid is the weakest and
which one is the strongest. (Remember: relative strength is not based on pH alone!)
acid
A
B
C
10.
+ H2O (l)
Initial concentration
0.10
0.000010
0.010
pH
3.0
6.9
3.6
Given that the K b of NH3 is 1.8 x 10- 5, calculate the K a of the ammonium ion, NH4+.
1) First write an equation showing that NH3 is a Bronsted base in water.
Complete the following:
NH3(aq) +
H2O(l)
=
2) Now write a K b expression for the above.
3) Multiply both the numerator and denominator by [H+].
4) Substitute Kw into your answer for #3,. Remember what Kw equals.
5) Write an equilibrium equation showing how NH4+ acts as an acid.
6) Write a KA expression for your equation in #5.
7) Write an expression for
8) Substitute your answer from #7 into your expression for #4
9) Now solve the main problem at the top of the page (Get KA for ammonium ion)
11. Spinoff Problem Calculate the percent of NH4+ molecules ionized (also known 14- 106 -as
percent dissociation )in a 0.10 M solution.(Use your KA from step 9)
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