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AcidsLesson 17
BuffersIndicators
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Buffer Solutions
Buffer solutions are equilibrium systems, which maintain a relatively constant pH when small amounts of acids or bases are added.
Acid Buffer Recipe
Weak Acid + Salt (conjugate)
HF NaF
Buffer Equation Same as weak acid
HF ⇋ H+ + F-
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HF ⇋ H+ + F-
[High] [low] [High]
[HF] is high and [H+] is low because HF is a weak acid.
[F-] is high due to adding the salt NaF.
This buffer solution can shift left or right and the [H+] and the pH remains fairly constant.
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Base Buffer Recipe
Weak Base + Salt (conjugate)
NH3 NH4Cl
Buffer Equation Same as weak base
NH3 + H2O ⇋ NH4+ + OH-
[High] [High] [Low]
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Fill in the blanks to get buffer solutions.Acid Base
HCN NaCN
H2CO3 NaHCO3
H3PO4 NaH2PO4
CH3COOH KCH3COO
NH4Cl NH3
NaHCO3Na2CO3
NaH2PO4Na2HPO4
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Buffer Application
1. Pick the two buffer solutions
A. NH4Cl and HCl
B. NaCl and NaOH
C. HCN and NaCN
D. NH4Cl and NH3
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2. Write the equations for the above two buffer solutions. Label the concentrations as high or low.
HCN ⇋ H+ + CN-
[High] [Low] [High]
NH3 + H2O = NH4+ + OH-
[High] [High] [Low]
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3. A few drops of 0.10 M NaOH is added to a buffer solution at pH = 5.00. The new pH could be:
A 4.98
B. 5.02
C. 8.05
D. 2.11
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Acid Base
Yellow Red
Shift Colour
At pH < 10.1 Left Yellow [HInd] > [Ind-]
At pH > 12.0 Right Red [HInd] < [Ind-]
At pH = 11.05 Middle Orange [HInd] = [Ind-]
HInd ⇋ H+ + Ind-Alizarin Yellow
Indicators
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@ the transition point
[HInd] = [Ind-]
Ka = [H+]
Ka = 10-pH
Ka = 10-11.05
= 9 x 10-12