Section 6.3

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Section 6.3Acidity, pH
How does concentration of acid affect the pH of a
sports drink?
2
A Review of Acids Bases
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Acids Arrhenius Definition

• Produce Hydronium ion (H3O1) in water
• Hydronium ion is water a hydrogen cation

By this definition, if an acid is to give a H1
to water, then all acids will have hydrogen as
the cation (first element written).
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How do Acids produce Hydronium?
water
acid
Hydrogen cation with some anion
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How do Acids produce Hydronium?
1
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How do Acids produce Hydronium?
1
Hydronium ion
Anion
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Bases Arrhenius Definition

• Bases produce the hydroxide ion in water

Hydroxide Ion
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Characteristics of Acids Bases
Bases
Acids
Produce H3O1 (hydronium ion) in water
Produce OH-1 (hydroxide ion) in water
Tastes sour
Tastes Bitter
React with active metals to form hydrogen gas
Feels slippery
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Strength versus Concentration
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Strong versus Weak Acids
How many hydronium ion anion pairs can you find?
3
How many intact acid molecules can you find?
1
Strong acid Most of the acid molecules have
donated the H1 to water
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Strong versus Weak Acids
How many hydronium ion anion pairs can you find?
1
How many intact acid molecules can you find?
3
Weak acid Only a few of the acid molecules have
donated the H1 to water
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Concentrated versus Dilute
solvent
solute
Lower concentration Not as many solute (whats
being dissolved) particles
Higher concentration More solute (whats being
dissolved) particles
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Combinations of Concentration Strength
Dilute
Concentrated
Strong
A lot of acid added most dissociates
Not much acid added, but most of whats there
dissociates
A lot of acid added, but most stays together
Not much acid added and most of what is there
stays together
Weak
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Acids and Bases as Electrolytes
Acids and bases dissociate into ions in water
Free-floating ions in water conduct electricity
Acids Bases are electrolytes
Strong acids and bases are strong
electrolytes Weak acids and bases are weak
electrolytes
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pH
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pH Scale

• Is a scale to measure the acidity of a sample

Chapter 6 will give more detail about how pH is
calculated!
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pH is a Logarithmic Scale
Logarithm The number of times a base must be
multiplied by itself to reach a given number
of multiples
Base
youre trying to reach
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Calculating pH
pH scale Logarithmic scale of the acidity of a
solution
The pH scale uses base 10
concentration in Molarity
pH has not units
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The - in the pH equation
Because pH is the negative log of concentration
of hydronium, as concentration increases, the pH
goes down.
The lowest pH is the highest concentration of
hydronium
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What does a log scale really mean?
Every change of 1 in pH shows a change of 10x in
concentration of hydronium
pH 4 3 2 1
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An example of calculating pH
Example Find the pH if the concentration of
H3O1 is 0.25 M
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An example of calculating pH
Example Find the pH if the concentration of
H3O1 is 0.25 M
pH 0.60
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An example of calculating hydronium
Example Find the H3O1 if the pH is 2.7
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An example of calculating hydronium
Example Find the H3O1 if the pH is 2.7
H3O1 0.0020 M
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Auto-ionization of Water

• Water will split into ions
• 2 H2O ? H3O1 OH-1
• Water will do this to make sure that at 25C the
  following is true
• H3O1 OH-1 1 10-14
• So if you know the hydronium concentration at
  25C (which can be found from pH), then you can
  also find the hydroxide concentration

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An example of calculating hydroxide
Example Find the OH-1 if the pH is 10.7
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An example of calculating hydroxide
Example Find the OH-1 if the pH is 10.7
H3O1 2.0 10-11 M
OH-1 0.0005 M
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Lets Practice 1
Example Find the pH if the concentration of
H3O1 is 2.5 10-5 M
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Lets Practice 1
Example Find the pH if the concentration of
H3O1 is 2.5 10-5 M
pH 4.6
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Lets Practice 2
Example Find the OH-1 if the pH is 3.6
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Lets Practice 2
Example Find the OH-1 if the pH is 3.6
H3O1 2.5 10-4 M
OH-1 4.0 10-11 M
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Lets Practice 3
Example Find the H3O1 if the pH is 11.2
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Lets Practice 3
Example Find the H3O1 if the pH is 11.2
H3O1 6.3 10-12 M