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ACIDS AND BASES – 2IONIZATION OF WATER,
Dr. Sapna Gupta
pH
IONIZATION OF WATER
Self-Ionization of Water
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
Base Acid Conj. Acid Conj. base
We call the equilibrium constant the ion-product constant, Kw.
Kw = [H3O+][OH−] At 25°C, Kw = 1.0 × 10−14
As temperature increases, the value of Kw increases.
Solutions can be characterized as
Acidic: [H3O+] > 1.0 × 10−7 M
Neutral: [H3O+] = 1.0 × 10−7 M
Basic: [H3O+] < 1.0 × 10−7 M
Dr. Sapna Gupta/Acids Bases pH 2
EXAMPLE: CALCULATION OF CONCENTRATIONS
Calculate the hydronium and hydroxide ion concentration at 25°C in
a. 0.10 M HCl
b. 1.4 × 10−4 M Mg(OH)2
Solution:
a. When HCl ionizes, it gives H+ and Cl−. So [H+] = [Cl−] = [HCl] = 0.10 M.
b. When Mg(OH)2 ionizes, it gives Mg2+ and 2 OH−. So 2[OH−] = 2(1.4 × 10−4 M) = 2.8 × 10−4 M.
Dr. Sapna Gupta/Acids Bases pH 3
CALCULATING
pH = –log[H3O+]
And inverse:
• When pH = 7, the solution is neutral ([H3O+] = 1.0 × 10−7 M).
• When pH < 7.00, the solution is acidic ([H3O+] > 1.0 × 10−7 M).
• When pH > 7.00, the solution is basic ([H3O+] < 1.0 × 10−7 M).
Dr. Sapna Gupta/Acids Bases pH 4
[ ] pH3H O 10
pH
EXAMPLE: CALCULATING
1) What is the pH of a solution that has a hydronium ion concentration of 6.5 x 105M?
2) What is the hydronium ion concentration of a solution with pH 3.65?
Dr. Sapna Gupta/Acids Bases pH 5
4.19pH
]Olog[HpH 3
]10 x log[6.54pH 5
4
3 2x102]O[H .
[ ] pH3H O 10
.[ ] 3 653H O 10
pH
, AND KW
• pOH can be calculated just like pH.
• KW is the hydronium ion and hydroxide ion concentrations; so pKW is the addition of pH and pOH.
• Next slide has all relationships…
Dr. Sapna Gupta/Acids Bases pH 6
]log[OHpOH
[ ] pOHOH 10
14.00pOHpHp w K
]][OHO[H3w
K
pH pOH
[H3O+] [OH−]
pH pOH
]O[H
101][OH
3
14
]Olog[H
pH
3
pOH10
][OH
pOH
log[OH ]
pH
3
10
]O[H
pOH14pH
pH14pOH
][OH
101]O[H
14
3
Dr. Sapna Gupta/Acids Bases pH 7
EXAMPLE -
1) What is the pOH of a solution that has a hydroxide ion concentration of 4.3 x 102M?
2) What is the hydroxide ion concentration of a solution with pOH 8.35?
Dr. Sapna Gupta/Acids Bases pH 8
371pOH .
]log[OHpOH
]10 x log[4.3pOH 2
[ ] pOHOH 10.[ ] 8 35OH 10
95x104][OH .
pOH
STRONG ACIDS AND BASES
• All strong acids and bases ionize completely; so their concentrations can be used to calculate the pH and pOH.
• For diprotic acids, e.g. sulfuric acid only the first ionization reactions is measured; on the contrary dihydroxide bases will ionize to give two hydroxides in solution.
Dr. Sapna Gupta/Acids Bases pH 9
EXAMPLE
• What is the pH of a 0.057 M solution of HBr?
Solution: HBr ionizes completely so the conc. of the acid can be used as H3O+.
Note: very low pH – strong acid.
• What is the pOH of a solution of 0.034 M solution of Ca(OH)2?
Solution: Ca(OH)2 ionizes to give 2 mols of OH-, so conc. has to be doubled.
Dr. Sapna Gupta/Acids Bases pH 10
]Olog[HpH 3
log[0.057]pH
1.24pH
HBr + H2O H3O Br +(l) (aq) (aq)(aq)
(aq) (aq)(aq) + OHCa2+
Ca(OH)2 2
[ ] . ( )( )
2
2
2 mol OHOH 0 034 M Ca OH x
1 mol Ca OH
]log[OHpOH
log[0.068]pOH
1.17pOH
M0.068][OH
KEY CONCEPTS
• Calculation of pH and pOH
• Calculation of hydronium and hydroxide ion conc.
Dr. Sapna Gupta/Acids Bases pH 11