Lecture 53 Acids and Bases III

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Lecture 53 - Acids and Bases III
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Organic Bases

• fish smell due to amines

Lemon Juice (citric acid)
salts (less volatile less smell)
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pH

• (potential Hydrogen)
• pH -log10H3O(aq)
• in pure water, pH -log10(10-7)
• - (-7)
• 7

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The pH scale
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pOH

• pOH -log10OH-(aq)
• but, H3O(aq) OH-(aq) Kw
• thus, OH-(aq) Kw / H3O(aq)
• thus, pOH -log10(Kw / H3O(aq))
• -log10 Kw log10H3O(aq)
• 14 - pH

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Calculating pH

• 1. pH of strong acid solutions
• e.g. 2.0 M HBr(aq)
• HBr(aq) H2O(l) ? H3O(aq) Br-(aq)
• H3O(aq) 2.0 M
• pH -log10(2.0)
• -0.30

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Note H2O dissociation is insignificant (in this
example)

• 2 H2O ¾ H3O(aq) OH-(aq)

strong acid drives equilibrium back
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Calculating pH

• 2. pH of weak acid solutions
• e.g. 2.0 M HF(aq)
• HF(aq) H2O(l) ¾ H3O(aq) F-(aq) Ka 7.2 x
  10-4
• 2 H2O ? H3O(aq) OH-(aq) Ka 1.0 x
  10-14

less significant reaction - thus H3O comes
from HF
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Calculating pH

• 2. pH of weak acid solutions
• e.g. 2.0 M HF(aq)

1. ASSUME HF 2.0 M
x2
2.
7.2 x 10-4
2.0
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Calculating pH

• 2. pH of weak acid solutions
• e.g. 2.0 M HF(aq)

3. x H3O F- Ö
2.0 x 7.2 x 10-4
0.038 M
4. pH -log10(0.038) 1.4
(2.0 M HBr had pH -0.30)
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Calculating pH

• 3. pH of strong base solutions
• e.g. 6 M KOH
• OH- 6.0 M
• pOH -log10(6.0) -0.78
• pH 14 - pOH 14.78

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Polyprotic Acids

• e.g. H2SO4, H2CO3, H3PO4, etc.

sulfuric carbonic phosphoric
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e.g. phosphoric acid

• H3PO4(aq) H2O(l) ¾ H3O(aq) H2PO4-(aq)
• H2PO4-(aq) H2O(l) ¾ H3O(aq) HPO4-2(aq)

7.5 x 10-3
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e.g. phosphoric acid

• H2PO4-(aq) H2O(l) ¾ H3O(aq) HPO4-2(aq)
• HPO4-2(aq) H2O(l) ¾ H3O(aq) PO4-3(aq)

H3O HPO4-2
Ka2
6.2 x 10-8
H2PO4-
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e.g. phosphoric acid

• since Ka1 gt Ka2 gt Ka3,
• H2PO4- gt HPO4-2 gt PO4-3
• and

most H3O comes from first dissociation
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e.g. phosphoric acid

• find concentrations of all species and the pH in
  0.10 M H3PO4(aq)
• H3PO4 H2O ¾ H3O H2PO4-
• Initial 0.10 0 0
• Final 0.10 - x x x

x2
7.5 x 10-3

0.10-x
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e.g. phosphoric acid
x2
7.5 x 10-3

0.10-x
solving, x H3O H2PO4- 0.0239 M pH
-log10(0.0239) 1.62
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e.g. phosphoric acid
H3O HPO4-2
Ka2
6.2 x 10-8
H2PO4-
equal!
6.2 x 10-8 M
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e.g. phosphoric acid

1.25 x 10-18 M
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e.g. carbonic acid

• CO2(g) H2O(l) ¾ H2CO3(aq)
• KH 0.032 mol L-1 atm-1
• (dissolution of atmospheric CO2(g))
• H2CO3(aq) KH x pCO2
• 0.032 mol L-1 atm-1 x (350 x 10-6 atm)
• 1.12 x 10-5 mol L-1

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e.g. carbonic acid

• but
• H2CO3(aq) H2O ¾ H3O(aq) HCO3-(aq)
• Ka1 4.3 x 10-7

H3O HCO3-
4.3 x 10-7
H2CO3
x2
4.3 x 10-7
(1.12 x 10-5 - x)
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e.g. carbonic acid

• solving, x H3O 2.00 x 10-6 M
• pH -log10(2.00 x 10-6)
• 5.70
• i.e. natural fresh water is naturally acidic

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e.g. carbonic acid
in limestone lined lakes...
CaCO3(s) ¾ Ca2(aq) CO3-2(aq)
HCO3-(aq) H2O(l) ¾ H3O(aq) CO3-2(aq)
H2CO3(aq) H2O ¾ H3O(aq) HCO3-(aq)
pH rises!