Acid / Base Chemistry
Acid & Base Strength & pH
pH of common materials
Strong vs. Weak Acids
Strong Acids
 A strong acid is one that ionizes completely in aqueous
solution.
HCl + H2O  H3O+ + Cl–
100%
Weak Acids
 A weak acid releases few hydrogen ions in aqueous solution –
it does not ionize completely.
HCN + H2O


H3O+ + CN–
???%
Strong Acids & Bases
Strong Acids
 HI
 HBr
 HCl
 HNO3
 H2SO4
 HClO4
 HClO3
Strong Bases
 NaOH
 KOH
 RbOH
 CsOH
 Ca(OH)2
 Sr(OH)2
 Ba(OH)2
Self-Ionization of Water
H2O + H2O  H3
+
O
+
[H3O+] = 1 x 10-7 M
[OH-] = 1 x 10-7 M
[H3O+] [OH-] = 1 x 10-14 M
–
OH
Hydronium vs. hydroxide concentrations
[H3O+] [OH-] = 1 x 10-14 M
Where do we get pH from?
 The pH of a solution is defined as the negative of the
common logarithm of the hydronium ion concentration,
[H3O+] or [H+].
pH = -log[H+]
example: if a solution has [H+]= 2.5 x 10−5
pH = −log[H+] = −log(2.5x10-5) = 4.6
pH Scale
 What is the pH of 0.050 M HNO3?
Nitric Acid is a strong acid, so it ionizes completely:
HNO3  H+ + NO3–
pH = -log[H+]
pH = -log[0.050]
pH = 1.3
C. Johannesson
pH is a Logarithmic Scale
 Every unit of pH
means there is 10x
more H+ ions!
 Stomach acid: pH 1
 Battery acid: pH 0
 Battery acid is 10x more
acidic than stomach acid!
pH is a Logarithmic Scale
 Coffee is pH 5
 Apple Juice is pH 3
 How much more acidic
is apple juice compared
to coffee?
 Apple juice is 2 pH units
more acidic than coffee, so
10 x 10 = 100 times more
acidic than coffee!
Equations you need to know!
pH = -log[H3O+]
pOH = -log[OH–]
pH + pOH = 14
[H3O+] = 10(–pH)
[OH–] = 10(–pOH)
[H3O+][OH–] = 1 x 10-14 M
pH Scale
 What is the pH of 0.030 M NaOH?
Sodium hydroxide is a strong base, so it ionizes completely:
NaOH  Na+ + OH–
pOH = -log[OH–]
pOH = -log[0.03] = 1.5
pH + pOH = 14
pH = 14 – pOH = 14 – 1.5
pH = 12.5
C. Johannesson
pH Scale
 What is the molarity of HBr in a solution that has a
pH of 4.4?
[H3O+] = 10(–pH)
[H3O+] = 10(–4.4)
[H3O+] = 4.0  10-5 M HBr
C. Johannesson
pH of Rainwater
across United States in 2001
Why is the eastern US more acidic?
http://nadp.sws.uiuc.edu/isopleths
What is acid rain?
Acid rain has a pH lower than 5.3
Dissolved carbon dioxide naturally lowers the pH
CO2 (g) + H2O  H2CO3  H+ + HCO3-
Atmospheric pollutants from combustion acidify rain a lot!
NOx : NO, NO2 + H2O …  HNO3
SOx : SO2, SO3 + H2O …  H2SO4
Marble + Hydrochloric Acid
 CaCO3 + 2HCl  CaCl2 + H2O + CO2
The photo on the left was taken in 1908. The effects of acid rain on the statue
are clear in the photo on the right, taken 60 years later, in 1968
Effects of acid rain
Limestone and acid rain
 Limestone is primarily CaCO3
 Neutralizes excess acid and adds buffers against pH changes
… just one little problem …