pH and pOH
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pH is the percent hydrogen in a
solution
pOH is the percent hydroxide in a
solution
In a neutral solution pH and pOH
are equal
pH Scale
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The pH scale runs from 0-14
At pH 7 pH and pOH are each 7
As the [H3O+] increases, the pH
drops
As the [OH-] increases the pH
goes up
pH Calculations
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pH = -log [H+]
pOH = -log [OH-]
pH + pOH = 14
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There are several ways to test pH
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Blue litmus paper (red = acid)
Red litmus paper (blue = basic)
pH paper (multi-colored)
pH meter (7 is neutral, <7 acid, >7 base)
Universal indicator (multi-colored)
Indicators like phenolphthalein
Natural indicators like red cabbage, radishes
Paper testing
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Paper tests like litmus paper and pH paper
„ Put a stirring rod into the solution and stir.
„ Take the stirring rod out, and place a drop of
the solution from the end of the stirring rod
onto a piece of the paper
„ Read and record the color change. Note what
the color indicates.
„ You should only use a small portion of the
paper. You can use one piece of paper for
several tests.
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Dilutions
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Concentrated solutions must often be
diluted
The relationship between concentration
and amount is inverse
To calculate the amounts of
concentrated solution and diluents
needed to make a specific
concentration
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C1V1 = C2V2
C = concentration
V = volume
Example
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If you need 200.0ml of a 2.2 M solution
of HCl and you only have 6.0 M HCl,
how would you prepare the solution?
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Calculate the amount of the concentrated
(6.0M) HCl needed
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C1V1 = C2V2
(6.0M)(V1) = (2.2M)(200.0ml)
V1 = 73.3ml
So of the 200.0ml of solution you are
making, 73.3ml will be the 6.0M HCl – the
rest will be water
To make the solution you would mix 73.3ml
of 6.0M HCl with 126.7ml of water
3
Example 2
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If you only have 23.0ml of
concentrated (12.0M) sulfuric
acid, how much 2.0M sulfuric
acid can you make?
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C1V1 = C2V2
(12.0M)(23.0ml) = (2.0M)(V2)
You could make 138ml of 2.0M
Sulfuric
Titrations
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In a titration, you are conducting
a neutralization reaction in order
to calculate the concentration of
an unknown acid or base using a
known concentration of acid or
base
In a titration, 2 burets are used for
accurate measurement. A measured
amount of the unknown is placed in a
beaker along with an indicator.
The solution of known concentration
is now added until the indicator
changes color (at the neutral point)
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Example
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What would be the
concentration of a sodium
hydroxide solution if it takes
25.4 ml of 1.3M hydrochloric
acid to neutralize 15.0 ml of
the base?
C1V1 = C2V2 or CaVa = CbVb
(1.3M)(25.4ml) = (M)(15.0ml)
„ 33.02 = 15.0 x M
„ 2.20M
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How do the H3O+ and OHbalance?
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You must account for the number of
hydronium and hydroxide ions
released on each side.
In the previous example it was not a
factor because HCl only forms one
hydronium and NaOH only forms one
hydroxide
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Example 2
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What would be the concentration of a
calcium hydroxide solution if it takes
52.8 ml of 1.1M phosphoric acid to
neutralize 25.0 ml of the base?
Ca(OH)2 forms 2 hydroxide ions
H3PO4 forms 3 hydronium ions
(1.1M)(52.8ml)(3) = (M)(25.0ml)(2)
174.24 = 50 M
3.48M
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