Acids

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Acids Bases Lesson 9 pH, pOH, Kb for Weak Bases
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Weak Bases calculations

• Helpful Hints
• Weak bases do NOT ionize 100
• There is an equilibrium state
• NEED.ICE tables!
• Kb cant be used directly from tableuse
• Kb kw/Ka

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You can calculate the Kb using the Ka from the
table and the equation below.   Ka x Kb 1.0 x
10-14 _at_ 25 oC     Kb Kw Ka(conjugate)  
or   Ka Kw Kb(conjugate)
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Weak Bases calculations

• Two types of questions
• -calculate pOH, or OH-
• -calculate Kb

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• Calculate the pH of 0.50 M NH3.
• Weak Base Need Kb(NH3) Kw
• Ka(NH4)
• 1 x 10-14
• 5.6 x 10-10 1.786 x 10-5
• NH3 H2O ? NH4 OH-
• I 0.50 M 0 0
• C -x x x
• E 0.50 - x x x

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Kb NH4OH- NH3 x2
1.786 x 10-5 0.50 x 0.002988 M
OH- pOH 2.52 pOH -LogOH- pH pOH
pKw 14.000 pH 11.48
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2. Calculate the pH of 0.20 M Na2CO3. Weak
Base Need Kb(CO32-) Kw Ka(HCO3-)
1 x 10-14 5.6 x
10-11 1.786 x 10-4 CO32- H2O
? HCO3- OH- I 0.20 M 0 0 C -x x x E
0.20 - x x x
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Kb HCO3- OH- CO32- x2
1.786 x 10-4 0.20 x 0.005976
M pOH 2.22 pH
11.78
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TRY calculate H, OH-, pH, pOH for a 0.20 M
solution of NH3.
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3. Calculate the pH of 0.20 M NaCl. NaCl is a
neutral salt pH 7.00
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What if asked to calculate Kb?

• Kb indicates weak bases, then use ICE tables.
• need to know s to plug in the equation.

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EX If the pH of 0.40 M NH3 _at_ 25 oC is 11.427,
calculate the Kb.
pH 11.427
pOH 2.573
OH- 10-2.573
OH- 0.002673 M at equilibrium line!!!
NH3 H2O ? NH4 OH-
I C E
0.40 0 0
0.002673 0.002673
- 0.002673
0.002673 0.002673
0.3973
NH4OH-
(0.002673)2
1.8 x 10-5
Kb

0.3973
NH3
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TOGETHER The pOH of a 0.50 M solution of the
weak acid HA is 10.64. what is Kb for A-?
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TRY A 0.600 M solution of the weak base
hydroxylamine, NH2OH, has a pH of 9.904. What is
Ka for NH3OH ?
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Next question

• Strong acid and a strong base
• Calculate excess
• No ICE tables

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4. 15.0 mL of 0.20 M HNO3 reacts with 40.0 mL of
0.20 M KOH, calculate the pH of the resulting
solution.
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Next question

• It says saturated solution equilibrium
• This is a solubility equilibrium- no ICE
• Remember, unit 3!

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4. Calculate the pH of a saturated solution of
Mg(OH)2. This is a solubility equilibrium- no
ICE Mg(OH)2(s) ? Mg2 2OH- s s 2s Ksp
Mg2OH-2 5.6 x 10-12 s2s
2 5.6 x 10-12 4s3 5.6 x
10-12 s 1.119 x 10-4 M
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2s OH- 2.237 x 10-4
M pOH 3.65 pH 10.35
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Be careful

• Are they strong or weak acids and bases?
• calculations differ! Use ICE tables for only weak
  combinations.
• Is it a Ksp question?
• Always understand the question BEFORE answering
  it.
• Remember ALL the equations!

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pH Equations You must know the following
equations, which are all based on the ionization
of water at 250 C!   H2O ? H OH-   Kw H
OH- 1.00 x 10-14     pH -LogH Or pOH
-LogOH-   H 10-pH OH- 10-pOH   pH
pOH pKw 14.000
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Homework

• p.153
• 84, 86, 87, 88,
• p.154
• 90, 91, 93.