Acid Base Strength and pH pOH
May 02, 2014
Strong vs. Weak Acids/Bases
Strong Acids/Bases:
· completely dissociate in solution
· Strong Acids: HCl, HClO 4 , HNO3 , HI, H2 SO 4 , HBr
HCl(aq) --> H+1(aq) + Cl-1(aq)
· Strong Bases: NaOH, KOH, Ca(OH) 2 , Ba(OH) 2
NaOH(aq) --> Na+1(aq) + OH-1(aq)
Weak Acids/Bases:
• do not completely dissociate in solution
· Weak Acids: HC 2 H3 O2 , HF, HCN, H 2 CO 3
HC2 H3 O2(aq) + H2 O(l) <---> C2 H3 O2 -1(aq) + H3 O+1(aq)
· Weak Bases: NH3 , CH3 NH2
CH3 NH2(aq) + H2 O(l) <---> CH3 NH3 +1(aq) + OH-1(aq)
Diprotic Strong Acid Dissociation...
Sulfuric Acid - H 2SO4
• The dissociation of the 1st H+1 is strong.
H2SO4 --> H+1 + HSO4-1
• no equilibrium
• The dissociation of the 2nd H+1 is weak.
HSO4-1 + H2O <---> H3O+1 + SO4-2
• written as an equilibrium
• has a K value
Acid Base Strength and pH pOH
May 02, 2014
Dilute vs. Concentrated
Acids & Bases
· the terms "weak/strong" are NOT the same as
"concentrated/dilute"
· weak & strong relate to the extent of
dissociation of an acid or base
· concentrated & dilute relate to the amount of
acid or base dissolved
**Molarity!!**
· Examples:
0.01 M HCl
2.5 M HC2H3O2
0.05 M NH3
10 M NaOH
dilute strong acid
concentrated weak acid
dilute weak base
concentrated strong base
Ion Product Constant for Water
(Kw)
Pure Water: Auto-ionization
H2O(l) + H 2O(l) <---> H 3O+1 (aq) + OH -1 (aq)
hydronium ion
same as H+1
Kw = [H3O+1 ][OH-1 ] = 1.0 x 10-14
If [H3O+1 ] = [OH-1 ]...
then [H3O+1 ] = [OH-1 ] = 1.0 x 10-7 M
Acid Base Strength and pH pOH
May 02, 2014
pH/pOH Scales
The pH Scale:
logarithmic scale that measures the [H+1] in solution
pH = - log[H+1]
0
ACID
7
BASE
14
** 1 unit change in pH = 10-fold change in [H+1]**
The pOH Scale:
logarithmic scale that measures the [OH-1] in solution
pOH = - log[OH-1]
Relationship:
pH + pOH = 14
pH of Some
Common Substances
3 Ways to Change [H +1 ] in solution...
change the molarity
use a diprotic or polyprotic acid (more H+1 ions)
use a stronger acid (more dissociation)
Acid Base Strength and pH pOH
May 02, 2014
Equations to Know...
To calculate pH:
pH = -log[H+]
To calculate pOH:
pOH = -log[OH-]
To calculate [H+]:
[H+] = 10-pH (2nd log)
To calculate [OH-]:
[OH-] = 10-pOH (2nd log)
Other equations:
pH + pOH = 14
Kw = [H3O+][OH-] = 1.0 x 10-14
pH Calculations
1. What is the [H3 O+1] of a 0.25M HNO3 solution?
What is the [OH-1]?
2. What is the [OH-1] of a 3.2 M HCl solution?
3. What is the pH of a solution with an [OH-1] =
5.3 x 10-8 M? Is the solution acidic or basic?
4. Fill in the following table...
[H+1]
pH
pOH
[OH-1]
1.4
3.2 x 10-6
M
A/B/N