Aqueous Solutions and pH

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Aqueous Solutions and pH
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Hydronium Ions and Hydroxide Ions

• Acids produce the hydronium (H3O) ion in water
• HCl(aq) H2O(l) ? H3O(aq) Cl-(aq)
• Bases produce the hydroxide ion (OH-) in water
• NaOH(aq) ? Na(aq) OH-(aq)

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Self-Ionization of Water

• Water produces both hydronium and hydroxide ions
  through self-ionization
• H2O(l) H2O(l) ? H3O(aq) OH-(aq)
• (Brackets are used to symbolize
  concentration in molarity)
• Kw ionization constant of water
• H3O 1 x 10-7M OH- 1 x 10-7M

Kw H3O OH- (Kw 1 x 10-14 M2)
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pH Scale
- A means of expressing acidity or basicity of a
solution.
http//www.mpcfaculty.net/mark_bishop/pH_range.jpg
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pH

• Expressing acidity or basicity in terms of H3O
  or OH- concentration can be difficult due to
  extremely small values.
• pH is a more convenient means of quantitatively
  describing acidity of a solution.
• pH stands for pouvoir hydrogene power of
  hydrogen.

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pH Calculations

• pH -logH3O

The logarithm (log) of a number is equal to the
power that 10 must be raised to equal the number.
Example log (1 x 10-7) -7
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pH Practice

• 1) What is the pH of a solution with a H3O
  1.0 x 10-7?

pH -log(1 x 10-7)
pH -(-7.0) 7.0

• 2) What is the pH of a solution with a H3O
  3.21 x 10-4?

pH -log (3.21 x 10-4)
pH - (-3.49) 3.49
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pOH Calculations Practice
pOH -logOH-

• 1) What is the pOH of a solution with a OH-
  1.0 x 10-9?

pOH -log(1 x 10-9)
pOH -(-9.0) 9.0

• 2) What is the pOH of a solution with a OH-
  6.74 x 10-8?

pOH -log (6.74 x 10-8)
pOH - (-7.17) 7.17
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Calculating Concentrations from pH pOH
H3O 10-pH
OH- 10-pOH
1) What is the H3O of a solution with a pH
12.3?
H3O 10-12.3
H3O 5.01x10-13M

• 2) What is the OH- of a solution with a pOH
  4.25?

OH- 10-4.25
OH- 5.62x10-5M
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Connection between hydronium hydroxide

• pH pOH 14

H3O OH- 1 x 10-14
Neutral solutions H3O OH- pH
7 Acidic solutions H3O gt OH- pH lt 7 Basic
solutions H3O lt OH- pH gt 7
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Summary of Equations

• pH -logH3O

pOH -logOH-
H3O 10-pH
OH- 10-pOH
pH pOH 14
H3O OH- 1 x 10-14
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Practice Chart
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Calculator Help
If you have a TI-30Xa such as the one shown, you
must enter the information in the formula
backwards.
For example What is the pH of a solution with a
H3O 3.21 x 10-4? 1) Enter 3.21EE-4 2)
Press log 3) Press /- key
1
3
2
pH - log (3.21 x 10-4)
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Calculator Help cont.
What is the H3O of a solution with a pH
12.3?
1
2
3
H3O 10 -12.3
1) Enter 12.3 2) Press /- key 3) Press 10x key
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Graphing and 2-line calculators

• Enter exactly as formula reads!