Strength of Vinegar by Acid-Base Titration

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Strength of Vinegar by Acid-Base Titration
Vinegar

• Final Exercise 105 points

CH3COOH
NaOH
Lye
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? QUESTIONS ?
How, and with what accuracy and precision can you
determine the concentration of a solution of NaOH?
Using that NaOH solution, with what accuracy and
precision can you determine the concentration of
a solution of acetic acid - by titration?
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From last week
This week
Concepts Strong/Weak Acids Acid
Dissociation/Ka Concentration End point
/ Equivalence point Titration curves
Logarithms Indicator Mole Relationships
pH pKa Titration
Primary Standard
Techniques Titration / Back-titration pH
Measurement
Weighing by Difference
Standardization
Preparing solutions of precise concentration
Apparatus Burets pH Meter
Pipet
Volumetric Flask
Analytical Balance
Remember the first exercise!
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Last exercise involved acid-base titration in
which concentration of NaOH was given.
In this exercise, you STANDARDIZE the NaOH.
Standardization Analytical determination of
purity or concentration of substance through
reaction with substance of verified composition
and purity (Primary Standard )
This STANDARDIZATION involves reaction of the
base, NaOH, with a PRIMARY STANDARD, the weak
acid
High purity Stable Not hygroscopic Not
efflorescent High solubility High molar mass
H
potassium hydrogen phthalate (KHP)
phthalic acid
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STOICHIOMETRY
KHP is a monoprotic weak acid. One available
proton.
K HC8H4O4- OH- ? C8H4O4 H2O K
pKa is 5.4
The stoichiometry of the reaction is
1 mol KHP ? 1 mol NaOH
Acetic acid, CH3COOH, is also a monoprotic weak
acid.
CH3COOH OH- ? CH3COO- H2O
1 mol CH3COOH ? 1 mol NaOH
Its pKa is 4.7
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An aqueous solution, can have H 0.1
and OH- 0.1 at the same time

1. True
2. False

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An aqueous solution, can have H 0.1
and OH- 0.1 at the same time
H and OH- are dependent on one another in
aqueous solutions.
H OH- 1.0 X 10-14
B False
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pKa 4.7
Equivalence Point
pH 8.5
Titration curve for Acetic Acid with NaOH looks
like this
H A- Ka
--------------- HA
HA H Ka
---------- A-
HA -log H - log Ka - log
------- A-
HA pH pKa - log
----------- A-
pH pKa
pH at equivalence point is 8.5
Half equivalence point
Phenolphthalein is a reasonable indicator
mL of NaOH added
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The pH at the equivalence point in the titration
of acetic acid with sodium hydroxide is

1. the pKa of acetic acid
2. ½ the pKa of acetic acid
3. 7
4. Larger than 7

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Equivalence Point
pH 8.5
D. Larger than 7
mL of NaOH added
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PROCEDURE
It does not matter in which order you A.)
Standardize NaOH, B.) Titrate Unknown C.)
Determine pH of unknown
TAs will assign order to minimize waiting

• A. STANDARDIZATION
• Prepare solution of known concentration of
  primary standard, KHP
• Weigh sample BY DIFFERENCE
• Dissolve fully in ERLENMEYER
• Bring to total volume in VOLUMETRIC FLASK

Must transfer ALL the solution
Web supplement
Follow manual procedure
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CALCULATIONS KHP Solution Weight of vial
KHP 15.4371 g Weight of vial remaining
KHP 12.3495 g Weight of KHP transferred
3.0876 g Volume of KHP Solution
0.2500 L
Weigh by difference!!!
Weight of KHP
250 mL
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CALCULATIONS KHP Solution Weight of vial
KHP 15.4371 g Weight of vial remaining
KHP 12.3495 g Weight of KHP transferred
3.0876 g Volume of KHP Solution
0.2500 L Molarity of KHP Solution (3.0876 g /
204.22 g/mol) / 0.2500 L 0.06047 M
If calculated concentration of KHP differs more
than 10 from 0.06 M, you have either made an
error in weight of KHP or made a calculation
error.
0.06 ? 0.006 or between 0.054 and 0.066
Molar Mass of KHP
Weight of KHP
Volume of Volumetric Flask
4 Sig Figs because of
?
?
?
0.015119 Mols of KHP
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PROCEDURE (contd)
Determine concentration of stock NaOH Solution
( Nominally 0.1 M )
Titrate
measured volumes of standard KHP solution (known
concentration)
with
measured volumes of NaOH Solution (unknown
concentration)
(Delivered from buret 1 )
(Delivered from buret 2 )
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In STANDARDIZATION, BOTH REAGENTS are delivered
by a BURET
KHP

• Can "BACKTITRATE
• I.e., if end point is overshot, can recover by
  adding more KHP and continuing titration, e.g.,

NaOH
Do at most 3 titrations for the standardization
1. Add measured volume of KHP(35mL) 2. Titrate
to Phenolphthalein Endpoint 3. If overshoot
endpoint, add more KHP 4. Titrate to
Phenolphthalein Endpoint again
Make sure you label the burets.
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Having determined concentration of KHP solution,
calculate volume of 0.1 M NaOH needed to react
with 35 mL of KHP.
35 mL of our KHP solution (0.06047 M) contains
Amt Concentration X Volume
Why do we make this calculation?
35 mL X 0.06047 mmol / mL 2.1 mmol of KHP
That will require 2.1 mmol of NaOH
So we will know when to begin to add the NaOH
slowly!
What volume of 0.1 M NaOH will contain 2.1 mmol
of NaOH?
m moles Molarity (mmol/mL) X Volume (m L)
Amt Concentration X Volume
2.1 mmol 0.1 (mmol/mL) X V (mL)
V 2.1 / 0.1 21 mL ? 20 mL
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How many mL of 0.10 M NaOH would 35 mL of 0.10 M
KHP require?

1. 25 mL
2. 35 mL
3. 0.006407 M
4. Whatever

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How many mL of 0.10 M NaOH would 35 mL of 0.10
M KHP require?
Stoichiometry is 1 ? 1 35 mL X 0.10 M 3.5
mmol KHP 3.5 mmol KHP ? 3.5 mmol NaOH 0.10 M
0.10 mmol/mL 3.5 mmol / 0.10 mmol/mL 35 mL So,
the answer is,
B 35 mL
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PROCEDURE - UNKNOWN
Unknowns are solutions of acetic acid in water.
We titrate aliquots of (undiluted) unknown. In
this exercise, aliquot is a 5 mL pipet-ful (5.00
? 0.01) of solution
Mandrake Root Extract

• Dilute unknown with distilled water ( 40 mL)
• Add Phenolphthalein as indicator
• Titrate unknown to phenolphthalein end point.

Remember you cannot backtitrate in this case
Since aliquots are identical, can track precision
of titration by calculating percent error
involumes of NaOH used.
Adding water doesnt change the amount of acetic
acid.
aliquots are fixed, repetitive fractions of a
solution
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E.g., suppose volumes of NaOH are
25.37
,25.84
, 24.93
Avg Vol (25.37 25.84 24.93) / 3
25.38 Avg Dev ( 0.01 0.46
0.45) / 3 0.31 Dev 100 X 0.31 /
25.38 1.2
Since all other numbers in subsequent
calculations are the same for any run, that will
be percent deviation in the final result.
Do at most 5 titrations for unknown (incl prelim)
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The darkest pink color is the best signal for the
equivalence point in the titration of acetic acid
with NaOH using phenolphthalein

1. True
2. False

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The lightest pink color is the best signal for
the equivalence point in the titration of acetic
acid with NaOH using phenolphthalein
A few drops after that point is reached, the pH
of the solution increases rapidly and the
solution gets as deeply colored as it is able.
On further addition of base, solution lightens
due to dilution.
B False
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KHP Unknown solutions are colorless. No
interference from beverage color.
Before end point
At end point
Just past end point
Way past end point
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CALCULATIONS NaOH STANDARDIZATION Molarity of
KHP Solution 0.06047 M KHP buret reading,
final 37.44 mL 38.77 KHP buret
reading, initial 3.68 mL
4.73 Volume of KHP titrated 33.76 mL
34.04 NaOH buret reading, final 28.73 mL
31.83 NaOH buret reading, initial 4.52 mL
7.88 Volume of NaOH used 24.21 mL
23.95 mmol of KHP titrated 33.76 mL X
0.06047 M 2.042 mmol 2.202 mmol of
NaOH used 2.042 mmol 2.202 Molarity
of NaOH 2.042 mmol / 24.21mL 0.08435 M
0.08526 M
From Part 1
Stoichiometry is 1 to 1
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CALCULATIONS NaOH STANDARDIZATION Molarity of
KHP Solution 0.06047 M KHP buret reading,
final 37.44 mL 38.77 KHP buret
reading, initial 3.68 mL 4.73 Volume
of KHP titrated 33.76 mL 34.04 NaOH
buret reading, final 28.73 mL 31.83 NaOH
buret reading, initial 4.52 mL
7.88 Volume of NaOH used 24.21 mL
23.95 mmol of KHP titrated 33.76 mL X
0.06047 M 2.042 mmol 2.058 mmol of
NaOH used 2.042 mmol 2.058 Molarity
of NaOH 2.042 mmol / 24.21mL 0.08435 M
0.08592 M
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Suppose the 3rd titration produces 0.08539 M
We have three values for our NaOH concentration.
Are they in reasonable agreement?
Average (0.08435 0.08592 0.08539) / 3
0.08522 M Avg Dev (0.00087 0.00070
0.00017) / 3 0.00058 M Pct Dev 100 X
0.00058 / 0.08522 0.68
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CALCULATIONS - UNKNOWN Volume of Unknown 5.00
mL 5.00 5.00 Concentration of NaOH
solution 0.08522 M NaOH buret, final 22.47 mL
21.16 23.72 NaOH buret, initial 3.15 mL
2.37 4.22 Volume NaOH used 19.32 mL
18.79 19.50 mmol of NaOH used 19.32 0.08522
1.646 mmol 1.585 1.645 mmol of Acetic Acid
titrated 1.646 mmol 1.585
1.645 Acetic Acid Concentration 1.646 / 5.00
0.329 M 0.317 0.329
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Stoichiometry is 1 to 1
3 Sig Figs
From Standardization
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CALCULATIONS - UNKNOWN Volume of Unknown 5.00
mL 5.00 5.00 Concentration of NaOH
solution 0.08522 M NaOH buret, final 22.47 mL
21.16 23.72 NaOH buret, initial 3.15 mL
2.37 4.22 Volume NaOH used 19.32 mL
18.79 19.50 mmol of NaOH used 18.79 0.08522
1.646 mmol 1.601 1.645 mmol of Acetic Acid
titrated 1.646 mmol 1.601
1.645 Acetic Acid Concentration 1.601 / 5.00
0.329 M 0.320 0.329
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CALCULATIONS - UNKNOWN Volume of Unknown 5.00
mL 5.00 5.00 Concentration of NaOH
solution 0.08522 M NaOH buret, final 22.47 mL
21.16 23.72 NaOH buret, initial 3.15
mL 2.37 4.22 Volume NaOH used 19.32
mL 18.79 19.50 mmol of NaOH
used 19.50 0.08522 1.646 mmol 1.601
1.662 mmol of Acetic Acid titrated
1.646 mmol 1.601 1.662 Acetic Acid
Concentration 1.662 / 5.00 0.329 M
0.320 0.332
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The concentration of the sodium hydroxide to be
used in computations in this exercise is the
value given on the stock solution container.

1. True
2. False

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The concentration of the sodium hydroxide to be
used in computations in this exercise is the
value given on the stock solution container. .
B False
The purpose of the standardization is to
determine the precise concentration of the NaOH
solution.
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Avg Conc of Acetic Acid (0.329 M 0.320
M 0.332 M) / 3 0.327 M Avg Deviation
(0.002 0.007 0.005) / 3
0.005 M Pct Deviation 100 X 0.005 / 0.327
1.5
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Read Record Burets to 2 DECIMAL PLACES
24.64 mL
Read Record Weights to 4 DECIMAL PLACES
4.6427 g
Begin each titration with buret reading between
0.00 and 5.00 mL
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ANALYSIS OF ERRORS
STANDARDIZATION Precision 1. WEIGH 3g of
solid KHP using the analytical balance ( ?0.0004
g ) Precision 100 X 0.0004 / 3.0000 0.01
2. PREPARE A SOLUTION of KHP using 250 mL
volumetric flask ( ? 0.05 mL ) Precision 100 X
0.05 / 250.00 0.02 3. TITRATE measured
volumes of KHP and NaOH using burets ( ? 0.05 mL
) Precision KHP 100 X 0.05 / 30.00 0.2
NaOH 100 X 0.05 / 30.00 0.2
Concentration of NaOH determined with
PRECISION of approximately 0.01 0.02 0.2
0.2 0.43
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ANALYSIS OF ERRORS
TITRATION OF UNKNOWN Precision 1. TRANSFER
aliquot of unknown using a 5 mL transfer pipet (
? 0.01 mL ) Precision 100 X 0.01 / 5.00 0.2
Note that 1 drop (0.05 mL) ? 1 2. TITRATE
aliquot using standardized NaOH, again using a
buret ( ?0.05 mL ) Precision (Buret)100 x 0.05 /
30.00 0.2 OVERALL PRECISION of
determination of concentration of unknown 0.2
0.2 0.4
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Suppose you over-titrate by 10 drops I.e.,
you go past the end point by 10 drops (0.5 mL).

How large an error in accuracy results?

• Unknowns range in concentration from 0.5 M to
  1.0 M.
• 5 mL of the unknown contains between 2.5 and 5.0
• mmol of acetic acid.
• That will require between 25 and 50 mL of 0.1 M
  NaOH

The error represented by 0.5 mL will range from
100 X 0.5 / 25 2 to 100 X 0.5 / 50 1
Errors in indentifying the end point are not a
major contribution to errors in the accuracy of
the concentration of the vinegar unknown.
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What about the pipet?
The volume of the sample you are using is 5.00
mL, delivered by pipet.
The intrinsic precision of the 5 mL pipet is
0.01 mL
100 X 0.01 / 5.00 0.2
BUT
The volume of a drop is 0.05 mL. ?Each drop
represents 100 X 0.05/5.00 1 error in
the volume delivered!
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PITFALLS TO AVOID

• Incomplete transfer of solid KHP into flask
  (caused by weighing using a watch glass or paper)
• Weigh by difference!
• 2. Failure to dissolve KHP completely
• Make sure no KHP particles remain undissolved
• 3. Incomplete transfer of KHP solution from
  Erlenmeyer to volumetric flask
• Transfer completely and rinse with distilled
  water from wash bottle
• 4. Failing to bring KHP solution to correct
  volume
• Use dropper for last 0.5 mL !

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5. Incomplete mixing of KHP solution in
volumetric flask Mix thoroughly both before and
after bringing to final volume 6. Improper use
of the pipet Make sure you use the pipet as
instructed 7. Contamination of unknown by
allowing contact with syringe while
pipeting Discard any sample for which this
happens 8. Over-titrating unknown Stop at
earliest detectable pink color
GRADE ON THIS EXERCISE WILL DEPEND ON both
Accuracy and Precision
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NEXT WEEK SUSB-054 Part 1 Gasometric
Determination of Sodium Bicarbonate in a
Mixture DO PRELAB to SUSB-054 - 1
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