1030 Experiment No. 7, Titration The Molarity of Acetic Acid in Vinegar

General Chemistry I
CHEM-1030
Laboratory Experiment No. 7
(Revised 05/21/2015)
Titration: The Molarity of Acetic Acid in Vinegar
Introduction
Titration is defined as the process of determining the concentration of a solution by comparing it to the
known concentration of another solution. In an acid-base titration, the point at which moles of acid are
equivalent to moles of base is the equivalence point. This is expressed algebraically as:
na(mol) = nb(mol)
(Equation 1)
In the titration of acetic acid, the active ingredient in vinegar, all reactants and products are colorless
and the equivalence point is invisible. A small amount of added acid-base indicator allows
visualization of the equivalence point. Phenolphthalein, the indicator used in this experiment, is
colorless in an acidic solution and turns pink when the acid is neutralized and the solution turns slightly
basic. (When a weak acid is titrated with a strong base such as NaOH, the solution at the equivalence
point is slightly basic due to hydrolysis of the weak acid anion.) The point at which the indicator turns
color is the endpoint. The visual endpoint may not exactly correspond to the equivalence point, but
selecting the correct acid-base indicator gives the closest possible match between the endpoint and the
invisible equivalence point.
The concentration of a base titrating solution can be determined in one of two ways:
1. By using the base solution to titrate a standard acid solution of precisely known concentration, such
as commercially available 0.100 M HCl.
2. By using the base solution to titrate a known amount of a “primary standard”, a solid acid that can
be weighed easily and that reacts with base in a known stoichiometric manner.
We will do the second procedure and use a solid acid primary standard to standardize the NaOH
solution. Requirements for a primary standard acid are:
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

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It must exist in very pure form, preferably as a crystalline solid.
It must be chemically stable, must not react with atmospheric oxygen or absorb water from the air.
It should have a high formula mass to minimize the effect of weighing errors.
It must react in a known manner with the solution to be standardized.
A commonly used primary standard for standardizing base solutions is the monoprotic weak acid
potassium hydrogen phthalate, HKC8H4O4, also called acid potassium phthalate. Its name may be
abbreviated KHP where P stands for phthalate, not phosphorus. The formula mass of KHP is 204.2234
g/mol. Verify this for yourself as an exercise. KHP reacts with sodium hydroxide in a one to one
molar ratio according to the equations:
KHC8H4O4(aq) + NaOH(aq)  NaKC8H4O4(aq) + H2O(l)
(Equation 2)
or
KHP(aq) + NaOH(aq)
 NaKP(aq) + H2O(l)
1
(Equation 3)
Equation 1 is the starting point of this exercise since moles of acid equal moles of base at the
equivalence point. In this case, moles of the solid acid, symbolized by na, are calculated from the mass
of weighed KHP (between 0.6 and 0.7 g) and the KHP formula mass, 204.223 g/mol as shown in
Equation 4.
n a (mol) 
mass acid(g)
formula mass(g / mol)
(Equation 4)
At the equivalence point, moles of acid equal moles of base, but moles of base solution, nb(mol), also
equal the base concentration times the volume of solution used (Equation 5).
na(mol) = nb(mol) = Mb(mol/L) x Vb(L)
(Equation 5)
You will make up an approximately 0.1 M NaOH solution from stock 6 M NaOH. You will then use
this base solution to titrate a known mass of the standard acid so you can calculate the actual molarity
of the base, Mb, using Equation 6. Then you will use the same base solution to titrate an unknown acid
solution (vinegar) and determine its molarity, Ma, using Equation 7. Base volumes will be measured
with a buret marked in 0.1 mL intervals and read to 0.01 mL The unknown acid volumes will be
dispensed with a 5mL or 10 mL pipette. For keeping track of significant figures, assume that the
delivered pipette volume is precise to 0.01 mL, that is, that the pipette delivers 10.00 mL of solution.
The concentration of the base titrating solution is calculated by combining Equations 4 and 5 into
Equation 6 and solving for the base concentration, Mb. Take into account that molar concentration is
expressed as mol/L while the buret measures base solution volume in milliliters.
mass acid(g)
= Mb(mol/L) x Vb(L)
formula mass(g / mol)
(Equation 6)
After you determine the exact NaOH solution molarity by titration with KHP, you will use that
concentration to determine the molarity of acetic acid in an unknown vinegar sample by solving for Ma
in Equation 7, derived by substituting n(mole) = M(mol/L) x V(L) into each side of Equation 1.
MaVa = MbVb
(Equation 7)
Note: The standardization titrations do not necessarily have to be performed before the vinegar
titrations. Either set of titrations may be done first, whichever is more convenient, depending on the
availability of balances and KHP supply bottles in the laboratory.
Experimental
Preparation of NaOH Titrating Solution: Be sure your safety goggles are properly fitted to your
face. Make an approximately 0.1 M NaOH solution by dilution from 6 M NaOH stock solution as
follows: Dispense about 11 mL of 6 M NaOH into a 500 mL Florence flask. Fill the flask halfway
with distilled water and swirl the flask for 15 s to mix the contents thoroughly. When the solution
appears completely homogeneous, fill the flask nearly to the bottom of the neck with distilled water for
a total volume of about 500 mL. Place a rubber stopper securely in the neck, support it with the palm
of your hand, invert the flask and swirl it for 5 s. Right the flask and repeat this step two more times to
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ensure the solution is uniform. If you see “swirls” of inhomogeneity, the solution is not yet well
mixed. Keep the approximatley 0.1 M NaOH sealed with the rubber stopper. Remember, the
concentration of this base solution is not well known. Its actual concentration will be determined by
titration. You will use this base solution for all subsequent titrations. Do not discard it until the end of
the experiment.
NaOH is never used as a primary titration standard because the compound cannot be weighed
accurately. NaOH pellets exposed to the air immediately absorb water and carbon dioxide so their
purity is always in doubt. Moreover, NaOH solutions undergo slow conversion to aqueous sodium
carbonate unless they are carefully protected from atmospheric CO2.
Filling Buret with Base Solution: Obtain a 50 mL buret. If the buret is clean and dry, there is no
need to wash the buret or rinse it with tap water. Insert a stopcock and tip into the buret bottom. If
they do not fit tightly, match a different stopcock with your buret. Rinse the buret twice with 5 mL
portions of your 0.1 M NaOH solution, drain the solution through the tip and discard it. Support the
buret vertically with a buret clamp over a sink so the top of the buret is at or below your eye level.
(Never dispense any liquid above the level of your face.) Fill the buret with NaOH solution nearly to
the top. Open the stopcock and run some base solution out rapidly while tapping the stopcock to
dislodge any bubbles hidden inside. Adjust the volume of base so that it is at, or slightly below, the
zero mark. It is not necessary to get the initial volume to exactly zero. Record the initial volume of the
NaOH solution to the nearest 0.01 mL. (Note: If the 50-mL buret is filled to the very top graduation,
the initial volume should be read as 0.00 mL, not 50.00 mL.)
NaOH Solution Standardization: Using small beakers or plastic weigh boats, weigh four 0.6 to 0.7 g
samples of KHP on an electronic balance to 0.001 g. Take advantage of the convenient electronic
balance tare function. Place a weighing container on the balance pan and push the tare or “T” button.
The balance display will read 0.000 g, showing the balance considers the container to have no mass.
The mass of any material you put in the container will be displayed directly, eliminating the need for
two separate weighings and a subtraction step to get the mass of the chemical. (If the balance display
reads to only two decimal places, inform the instructor.)
Quantitatively transfer each weighed KHP sample to a labeled, clean, but not necessarily dry 125 or
250 mL Erlenmeyer flask. Pour the solid through a wide-stem funnel into the Erlenmeyer flask and
wash all traces of the solid out of the weighing container and funnel with distilled water from your
wash bottle. Add about 50 mL of distilled water and two drops of phenolphthalein indicator to each
Erlenmeyer flask and swirl briefly. (The KHP will not dissolve completely until you start adding
NaOH solution.) Titrate each KHP solution with NaOH solution as follows:
For easy visualization of the endpoint, place a sheet of white paper on the ring stand base under the
titration flask. Record the initial buret reading. Lower the buret so its tip is inside the Erlenmeyer
flask and run base solution rapidly into the flask. If you are right handed, grip the neck of the
Erlenmeyer flask with the fingers of your right hand and swirl the solution rapidly to mix the acid and
base. Meanwhile operate the stopcock with your left hand. As the endpoint approaches, all the solid
KHP will dissolve. The color change from clear to pink will persist longer where the base enters the
solution. The final addition of base should be slow, one drop at a time to avoid overtitration. Rinse the
upper flask walls several times during the titration with a few milliliters of distilled water from your
wash bottle to ensure all the acid and base mix in the flask. Distilled water contains equal (and almost
negligible) amounts of hydrogen and hydroxide ion and will not affect the titration results. You may
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also rinse the buret tip to remove hanging drops of base solution. Do not rinse excessively because that
may remove sodium hydroxide solution from inside the tip.
The titration endpoint occurs when the solution changes from colorless to a persistent pale pink color.
Sometimes less than half a drop of base brings this about. When you believe you are near the endpoint,
transfer less than one drop of base from the buret tip to the wall of the flask, rinse it down with distilled
water and swirl. Repeat this step until you reach the visible endpoint. After you read the final NaOH
volume, you may discard each titrated solution. Do several more titrations until you have three that
agree to within a few percent. Since your KHP masses are likely to be different, check for consistency
of your titrations by dividing the volume of base solution dispensed by the KHP mass (no units
necessary). Write the calculated ratios with the correct number of significant figures in the blank space
to the right of the titration data. This volume/mass ratio should be nearly constant for all titrations.
If you add base solution beyond what produces the first pale pink color, you have overtitrated. A pale
pink endpoint is best; medium pink is acceptable. Deep pink means you added too much base. Do not
use overtitrated results in your calculations. If you use more than 24 mL of base solution for a single
titration, fill the buret before the next titration so you will not run out of base before you finish.
Titration of Unknown Acid: Obtain an unknown vinegar solution and record its number on your data
sheet. Rinse a 5 or 10 mL pipette twice with a small amount of your unknown acid and discard the
rinsate down the drain. Pipette 5 or 10 mL aliquots of your unknown acid into four separate clean (but
not necessarily dry) Erlenmeyer flasks as follows: Fill the rinsed pipette by suction using a rubber bulb
until the acid solution level is about 2 cm above the upper stem fill line. Never pipette by mouth. Let
in small amounts of air with the bulb or your finger tip until the bottom of the meniscus just touches
the fill line. Move the pipette to an Erlenmeyer flask and drain the pipette into the flask with the
pipette tip held against the inside of the neck. Wait 2 s after the flow stops, touch the pipette tip to the
surface of the liquid and remove the pipette from the flask. Do not blow out the liquid remaining in the
tip. Volumetric pipets are calibrated to deliver, not to contain, the specified amount. Add two drops
of phenolphthalein indicator to each flask and titrate each solution with your standard NaOH solution
to a persistent pale pink endpoint. Repeat the titrations until you have three reliable results (pale pink
endpoints with base volumes that agree closely) from which to calculate the average molarity of your
acetic acid sample. (Since the volume of unknown acid solution (and therefore the moles of acid) you
pipette is the same each time, the NaOH solution volumes should be the same for each titration. There
is no need to calculate a base volume to acid ratio as you did for the standardization trials.)
Cleanup: When you finish titrating, remove the stopcock from the buret. Rinse the buret, stopcock
and tip thoroughly with tap water. Sodium hydroxide solution corrodes glass and will etch the buret if
allowed to remain inside. After you rinse the buret, your instructor may tell you to place the buret in a
special buret cleaner. Return your pipette and unknown bottle to the laboratory supply cart. Wipe up
any spilled KHP from the balance and surrounding area. Wipe your benchtop down with a wet sponge
and wash your hands before you leave the room.
Safety: The stock 6M NaOH solution is extremely corrosive and dangerous to the eyes. Chemical
splash goggles and a waterproof apron must be worn at all times during this and all chemistry
experiments, from the very beginning to the very end of the time you spend in the laboratory.
Waste Disposal: All spent titration solutions are neutralized and may be safely rinsed down the drain
with water. Your instructor will tell you how to dispose of unused 0.1 M NaOH titrating solution.
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General Chemistry I
CHEM-1030
Laboratory Experiment No. 7
Titration of Vinegar (The Molarity of Acetic Acid in Vinegar)
Report
Clearly show the equations, setups and steps for each calculation.
1.
For each of your three best base standardization titrations, calculate the moles of KHP you
weighed out.
(3 points)
2.
For each of your three best base standardization titrations, solve Equation 6 to calculate the
molarity of your NaOH titrating solution.
(6 points)
3.
From your three best trials, calculate the average molarity of the NaOH titrating solution.
(2 points)
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4.
Calculate the average volume of NaOH solution used in the three most consistent titrations of
your unknown acetic acid (vinegar) solution.
(1 point)
5.
Write your Unknown Vinegar Solution Number here:__________
Use Equation 7 to calculate the molarity of your unknown acetic acid solution.
(4 points)
6.
Calculate the mass of sodium hydroxide in one liter of your NaOH titrating solution. (2 points)
7.
Calculate the percent (w/v) concentration of your unknown acetic acid, that is, the mass of
acetic acid in 100 mL of solution.
(2 points)
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General Chemistry I
CHEM-1030
Data Page for Laboratory Experiment No. 7
Titration (The Molarity of Acetic Acid in Vinegar)
Standardization Titration # 1
Mass of KHP
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
Standardization Titration # 2
Mass of KHP
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
Standardization Titration # 3
Mass of KHP
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
Standardization Titration # 4
Mass of KHP
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
Standardization Titration # 5
Mass of KHP
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
Standardization Titration # 6
Mass of KHP
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
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General Chemistry I
CHEM-1030
Data Page for Laboratory Experiment No. 7
Titration (The Molarity of Acetic Acid in Vinegar)
Unkown Vinegar Solution Number__________________________
Vinegar Titration # 1
Volume of Pipetted Vinegar
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
Vinegar Titration # 2
Volume of Pipetted Vinegar
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
Vinegar Titration # 3
Volume of Pipetted Vinegar
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
Vinegar Titration # 4
Volume of Pipetted Vinegar
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
Vinegar Titration # 5
Volume of Pipetted Vinegar
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
Vinegar Titration # 6
Volume of Pipetted Vinegar
Final Volume of NaOH
Initial Volume of NaOH
NaOH Titrating Volume
Endpoint Color
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General Chemistry I
CHEM-1030
Laboratory Experiment No. 7
Titration (The Molarity of Acetic Acid in Vinegar)
Prestudy
1. A 0.708 gram KHP sample requires 11.98 mL of a NaOH solution to neutralize it using
phenolphthalein as an indicator. Use Equation 6 to calculate the NaOH molarity. Show
your setup
(5 points)
2. A 10.00 mL sample of acetic acid solution is titrated with 30.87 mL of 0.1274 M NaOH
solution. Calculate the molarity of the acetic acid solution. Show your setup.
(3 points)
3. What is the smallest marked division on the buret you will be using? To what precision is
the buret properly read?
(2 points)
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