Acid Base Titrations

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Experiment 7
Acid Base Titrations
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What are we doing in this experiment?
Determine the concentration of acetic acid in
vinegar. To follow the ionization of
phosphoric acid with the addition of NaOH and
plot a titration curve.
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What is acid-base titration?
A TITRATION WHICH DEALS WITH A REACTION
INVOLVING ACID AND A BASE.
What is a titration?
The act of adding standard solution in small
quantities to the test solution till the reaction
is complete is termed titration.
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What is a standard solution?
A standard solution is one whose concentration
is precisely known.
What is a test solution?
A test solution is one whose concentration is to
be estimated
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Titration of Vinegar against NaOH
Vinegar is an acetic acid solution of certain
concentration
So a titration of vinegar against NaOH
actually means, a reaction between acetic acid
and NaOH.
HC2H3O2(aq) NaOH(aq) ? NaC2H3O2
H2O(l)
Sodium hydroxide
Sodium Acetate
Acetic acid
water
water
Base
Acid
salt
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Titration of Vinegar against NaOH
HC2H3O2(aq) NaOH(aq) ? NaC2H3O2
H2O(l)
CH3COOH(aq) NaOH(aq) ? CH3COONa
H2O(l)
1 molecule
1 molecule
1 molecule
1 molecule
1 mole
1 mole
1 mole
1 mole
1 mole CH3COOH 1 mole NaOH
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Titration of Vinegar against NaOH
In this experiment, we are trying to find the
CH3COOH in vinegar. So we have to know
the NaOH accurately first, before finding the
CH3COOH in Vinegar.
Finding the NaOH should be pretty easy
right?!! Ya! Kind of ..
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How hard is it to make 500 mL of 0.1 M NaOH?
Should not be that hard right?!!
Calculate the weight of NaOH (2.0 g)
Weigh out the NaOH on the balance
Dissolve it in 500 mL of water in a volumetric
flask
Hold on! We have a problem here!!
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So what is your problem?
I dont have a problem but Mr. NaOH seems to have
a problem here
NaOH is hygroscopic..
What do you mean by hygroscopic?
It absorbs moisture. NaOH absorbs moisture from
air.
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What if NaOH is hygroscopic?
Let us say, for the problem at hand, we need 2.0
g NaOH to make a 0.1 M NaOH solution. By the time
we weigh out the NaOH for our solution, it would
have absorbed moisture. So the total weight of
2.0 g is not all due to NaOH. It has some
contribution from the water that the NaOH
absorbed.
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What if NaOH is hygroscopic?
Since the total weight of 2.0 g is not all due to
NaOH, the concentration of the 0.1 M NaOH that
we are trying to make is not going to be What we
expect it to be.
We will not know the accurate concentration of
the NaOH solution, just by dissolving 2.0 g of
NaOH in 500 mL of water.
But the requirement for a titration is that
we know the concentration of at least one of
the Solutions very precisely.
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How do we find the NaOH precisely?
Through standardization
What is standardization?
It is just a technical term for doing a titration
using a primary standard to find the precise
concentration of a secondary standard.
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What is a primary standard?
A primary standard should possess the following
qualities (i) It must be available in very pure
form (ii) It should not be affected by exposure
to moisture or air. (iii) It should maintain its
purity during storage. (iv) The reactions
involving the primary standard should be
stoichiometric and fast. (v) It should have high
molecular weight.
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Which primary standard are we going to use?
Potassium hydrogen phthalate, abbreviated as KHP.
Remember!! KHP is not the molecular formula
for Potassium hyrogen phthalate. It is just an
abbreviation. So when calculating the molecular
weight of KHP, do not add up the atomic weights
of K, H and P.
Potassium Hydrogen Phthalate, KHC8H4O4
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Standardization
KHC8H4O4(aq) NaOH(aq) ? KNaC8H4O4
H2O(l)
water
Base
Acid
salt
1 molecule
1 molecule
1 molecule
1 molecule
1 mole
1 mole
1 mole
1 mole
1 mole KHC8H4O4 1 mole NaOH
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Standardization
Vfinal- Vinital Vused (in mL)
Vinitial
moles of KHP Moles of NaOH
moles of KHP MNaOH VNaOH
0.1 M NaOH
Vfinal
moles of KHP MNaOH Vused
End point Pale Permanent Pink color
250mL
250mL
250mL
KHP H2O 2-3 drops of phenolphthalein
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Titration of Vinegar vs. NaOH
Vfinal- Vinital Vused (in mL)
Vinitial
moles of acetic acid
Moles of NaOH
moles of acetic acid MNaOH VNaOH
0.1 M NaOH
Vfinal
moles of acetic acid MNaOH Vused
End point Pale Permanent Pink color
250mL
250mL
250mL
vinegar H2O 2-3 drops of phenolphthalein
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Hydrolysis of salts formed from strong bases and
weak acids
NaCH3COO (CH3COO-)
NaOH CH3COOH SB WA
HA(aq) OH-(l) H2O
A-(aq)
conjugate acid
conjugate base
WA
SB
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Hydrolysis of salts formed from strong bases and
weak acids
A-(aq) H2O (l) HA
OH-(aq)
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Hydrolysis of salts formed from strong bases and
weak acids
Multiplying and dividing the numerator and
denominator by H3O
Rearranging the equation
(1)
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Hydrolysis of salts formed from strong bases and
weak acids
Remember!!
H2O (l) H2O(l) H3O
(aq) OH-(aq)
H2O(l) H (aq)
OH-(aq)
(2)
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Hydrolysis of salts formed from strong bases and
weak acids
Remember!!
For a monoprotic weak acid (HA) dissolved in
water,
HA(aq) H2O(l) H3O(aq) A-(aq)
conjugate acid
conjugate base
acid
base
(3)
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Hydrolysis of salts formed from strong bases and
weak acids
(1)
(2)
(3)
Substituting 3 and 2 in 1
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pH at the equivalence point
HA(aq) OH- (l) A- H2O(aq)
A-(aq) H2O (l) HA
OH-(aq)
At equivalence point, HA OH-
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pH at the equivalence point
Now we make 2 substitutions in the above equations
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pH at the equivalence point
At equivalence point, moles of A- ? moles of HA
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pH at the equivalence point
Taking log on both sides and multiplying both
sides of the equation by -1
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Titration curve of phosphoric acid, H3PO4
Phosporic acid is a triprotic acid
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Titration curve of phosphoric acid, H3PO4