Acid Base Titrations Experiment #7
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Acid Base Titrations
Experiment #7
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.
What is acid-base titration?A TITRATION WHICH DEALS WITH A REACTION INVOLVING ACID AND ABASE.
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.
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
Titration of Vinegar against NaOH
Vinegar is an acetic acid solution of certain concentration
So a titration of vinegar against NaOH actuallymeans, a reaction between acetic acid and NaOH.
HC2H3O2(aq) + NaOH(aq) NaC2H3O2 + H2O(l)
Acetic acid Sodium hydroxide
Sodium Acetate
water
Acid Base salt water
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
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 …..
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!!
So what is your problem?
I don’t have a problem but Mr. NaOHseems to have a problem here
NaOH is hygroscopic..
What do you mean by hygroscopic?
It absorbs moisture. NaOH absorbs moisture from air.
What if NaOH is hygroscopic?
Let us say, for the problem at hand,we need 2.0 g NaOH to make a 0.1 MNaOH 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.
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 concentrationof the NaOH solution, just by dissolving 2.0 g of NaOH in 500 mL of water.
But the requirement for a titration is that weknow the concentration of at least one of theSolutions very precisely.
How do we find the [NaOH] precisely?
Through standardization
What is standardization?
It is just a technical term for doinga titration using a primary standard to find the precise concentration of a
secondary standard.
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.
Which primary standard are we going to use?
Potassium hydrogen phthalate, abbreviated as KHP.
Remember!! KHP is not the molecular formula forPotassium 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.
COOH
C-H
COOKCH
HC
HC
Potassium Hydrogen Phthalate, KHC8H4O4
KHC8H4O4(aq) + NaOH(aq) KNaC8H4O4 + H2O(l)
Standardization
Acid Base salt water1 molecule 1 molecule 1 molecule 1 molecule
1 mole 1 mole 1 mole 1 mole
1 mole KHC8H4O4 ≡ 1 mole NaOH
250mL 250mL 250mL
Vinitial
Vfinal
End point:Pale PermanentPink color
“ 0.1 M NaOH ”
Vfinal- Vinital= Vused (in mL)
Standardization
KHP + H2O+ 2-3 drops of phenolphthalein
moles of KHP = Moles of NaOH
moles of KHP = MNaOH × VNaOH
moles of KHP = MNaOH × Vused
used
NaOH V
KHPofmolesM
250mL 250mL 250mL
Vinitial
Vfinal
End point:Pale PermanentPink color
“ 0.1 M NaOH ”
Vfinal- Vinital= Vused (in mL)
Titration of Vinegar vs. NaOH
vinegar + H2O+ 2-3 drops of phenolphthalein
moles of acetic acid = Moles of NaOH
moles of acetic acid = MNaOH × VNaOH
moles of acetic acid = MNaOH × Vused
)(LV
NaOHofmolesM
vinegar
acidacetic
Hydrolysis of salts formed fromstrong bases and weak acids
NaCH3COO (CH3COO-)
NaOH + CH3COOH SB WA
HA(aq) + OH-(l) H2O + A-(aq)
WA conjugateacidSB conjugate
base
Hydrolysis of salts formed fromstrong bases and weak acids
A-(aq) + H2O (l) HA + OH-(aq)
eqb
eqbeqb
b A
OHHAK
A
OHHAKb
Hydrolysis of salts formed fromstrong bases and weak acids
A
OHHAKb
Multiplying and dividing the numerator and denominator by [H3O+]
OHOH
A
OHHAKb
3
3
Rearranging the equation
1
3
3
OHOH
OHA
HAKb (1)
Hydrolysis of salts formed fromstrong bases and weak acids
Remember!!
H2O (l) + H2O(l) H3O+ (aq) + OH-(aq)
H2O(l) H+ (aq) + OH-(aq)
OHOHKwwaterofproductIonic 3,
14
3 101)25( OHOHCKw
(2)
Hydrolysis of salts formed fromstrong bases and weak acids
Remember!!For a monoprotic weak acid (HA) dissolved in water,
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
acid conjugateacidbase conjugate
base
eqbeqbeqb
a HA
AOHK
3
HA
AOHKa
3
AOH
HA
Ka 3
1(3)
Hydrolysis of salts formed fromstrong bases and weak acids
1
3
3
OHOH
OHA
HAKb (1)
OHOHKwwaterofproductIonic 3, (2)
AOH
HA
Ka 3
1(3)
Substituting 3 and 2 in 1
a
ww
a
b KKK
KK
11
wab KKK
pH at the equivalence point
HA(aq) + OH- (l) A- + H2O(aq)
A-(aq) + H2O (l) HA + OH-(aq)
eqb
eqbeqb
b A
OHHAK
A
OHHAKb
At equivalence point, [HA] = [OH-]
AOH
Kb
2
AOH
Kb
2
pH at the equivalence point
Now we make 2 substitutions in the above equations
OHK
OH w
3 a
wb KK
K
2
3
2
OHA
KKK w
a
w
AKKOH aw
1
2
3
pH at the equivalence point
AKKOH aw
1
2
3
AKK
OH aw3
At equivalence point, moles of A- moles of HA
HA
eequivalencatsolution
CLV
HAofmolesA
)(
HA
aw
CKK
OH 3
pH at the equivalence point
HA
aw
CKK
OH 3
Taking log on both sides and multiplying both sides of the equation by -1
HA
aw
CKK
LogOHLog 3
21
HA
aw
CKK
LogpH
HA
aw
CKK
LogpH21
Titration curve of phosphoric acid, H3PO4
Phosporic acid is a triprotic acid
Titration curve of phosphoric acid, H3PO4