Experiment 10
Name: _________________________
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Ti TRATION: ANALYSIS OF VINE Ga R
In this experiment, you will learn the concept and technique of titration. You will first
standardize a sodium hydroxide (NaOH) solution. This solution will then be used to determine
the concentration of acetic acid in commercial vinegar in terms of molarity and mass percent.
Titration is a technique used to determine the concentration of a solution by allowing it
to react with a solution of known concentration. For this experiment, you will determine the
concentration of acetic acid in vinegar by neutralizing it with a NaOH solution of known
concentration. In this case, vinegar is called the analyte and NaOH is called the titrant or the
standard.
In an acid-base titration, the titrant is dispensed slowly from a buret into a flask
containing the analyte and the indicator. The indicator signals the reaction completion by
changing colors when its environment switches between acidic and basic. In this experiment,
phenolphthalein is used as indicator; it is colorless in acid and pink in base. As soon as a change
in color is observed, titration is stopped. The endpoint, the point when the indicator changes
color, serves as a visually detectable close approximation to the equivalence point, the point
when the amounts of acid and base are stoichiometrically equal. One must ensure that this
approximation holds by carefully performing the titration, adding the titrant dropwise while
closely watching for the endpoint. The last drop of the titrant added must be the drop that
brought about the change in color. The concentration of the analyte is determined from the
amount of titrant added required to reach the endpoint.
Standardization of NaOH
Since titration is a quantitative method, the concentration of the standard must be
known with good certainty by standardization. To standardize the NaOH solution, it will be
neutralized by potassium hydrogen phthalate (abbreviated KHP) in a titration analysis. KHP is a
primary standard, being used to calibrate the standard NaOH solution. The equation for this
acid-base reaction is:
KHC8H4O4(aq) + NaOH(aq)  KNaC8H4O4(aq) + H2O(l)
A known mass of KHP crystals is added to an Erlenmeyer flask then dissolved in water.
Phenolphthalein is then added to the flask and it is colorless in acidic solution. The NaOH
solution is dispensed from the buret until the endpoint is reached when the solution turns pink.
Consider 0.968 g of KHP (molar mass = 204.23 g/mol) dissolved in water requiring
20.06 mL of the NaOH solution to reach phenolphthalein endpoint. The concentration of the
NaOH solution is calculated as follows:
mol KHP
1 mol NaOH
0.968 g KHP (
)(
) = 0.00474 mol NaOH
204.23 g KHP
1 mol KHP
1
M=
0.00474 mol NaOH
= 0.236 M NaOH solution
0.02006 L solution
By standardization, it is now known that the concentration of the NaOH solution is 0.236 M.
This solution can now be used as a standard in the analysis of vinegar.
Analysis of Vinegar
Vinegar is an aqueous solution of acetic acid, HC2H3O2. In this experiment, a sample of
commercial vinegar is titrated with a standard NaOH solution to phenolphthalein endpoint to
determine the molar concentration of acetic acid in vinegar and the mass percent. The chemical
equation is:
HC2H3O2(aq) + NaOH(aq)  NaC2H3O2(aq) + H2O(l)
For example, if a 10.00-mL vinegar sample is titrated with 28.75 mL of the 0.236 M
standard NaOH solution, the molarity of HC2H3O2 in the vinegar is calculated as follows:
0.236 mol NaOH mol HC2 H3 O2
0.02875 L NaOH soln (
)(
) = 0.00679 mol HC2 H3 O2
L soln
mol NaOH
M=
0.00679 mol HC2 H3 O2
= 0.679 M HC2 H3 O2 solution
0.01000 L solution
Mass percent concentration is given by:
mass % =
mass solute
× 100%
mass solution
To convert the concentration in molarity to mass percent, first consider a 1-L volume of the
solution. A liter of a 0.679-M HC2H3O2 solution contains 0.679 moles of the solute HC2H3O2.
The number of moles of HC2H3O2 is converted to mass using the molar mass:
0.679 mol HC2 H3 O2 (
60.0 g HC2 H3 O2
) = 40.74 g HC2 H3 O2
mol HC2 H3 O2
The mass of the 1-L solution is calculated using the density of vinegar, which is 1.01 g/mL:
1000 mL solution (
1.01 g solution
) = 1010 g solution
mL solution
The mass % is:
mass % =
40.74 g HC2 H3 O2
× 100% = 4.03 %
1010 g solution
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PROCEDURE
I. Standardization of the NaOH solution
A. Preparation of the KHP Solution
1. Place a clean 125-mL Erlenmeyer flask on a weighing balance then press “tare” to zero
the readout.
2. Add a little more than 1 g KHP into the flask. Record the mass of the KHP.
3. Using a squirt wash bottle, add ~25 mL of distilled water into the flask, directing water to
wash cystals that may be sticking to the walls of the flask.
Note: The volume of the water added to the flask does not need to be known accurately
since the calculation will be based on the mass of the KHP used in the analysis.
4. Add a drop of phenolphthalein indicator to the flask.
5. Add a magnetic stir bar to the flask and stir the solution on a stirring plate. Note that you
are using a combination heat/stir plate; use only the stirring function. Keep stirring as
you prepare your buret in Part IB allowing all the crystals to dissolve.
B. Preparation of the Buret
1. Obtain ~100 mL NaOH solution in a dry, clean beaker. You will use this beaker (and a
funnel if you find it necessary), to transfer NaOH solution to the buret. You may refill
the beaker with NaOH as needed.
2. Before using a buret for analysis, it needs to be conditioned to minimize errors due to
contaminants by rinsing the inside walls of the buret with the titrant, the NaOH solution.
Start with the stopcock of the buret closed, or in horizontal position.
3. Fill the buret halfway with the titrant (NaOH solution). Empty the buret by slowly
pouring out the solution into a waste beaker while turning the buret so that the solution
makes contact with all the inner surface of the buret.
4. Clamp the buret to the stand, making sure it is perfectly vertical. Fill with NaOH solution
just above the zero mark and place the waste beaker below the buret tip. Open the
stopcock to vertical position by turning it 90°. Drain some solution to fill the buret tip,
from below the stopcock to the very tip, making sure there are no air gaps or bubbles.
Note that the stopcock can control how fast the solution flows between the horizontal
and vertical positions. The buret is now ready for analysis. At this time, the liquid level
does not need to be at the zero mark; it just needs to be within the graduations.
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C. Titration of the Standard NaOH Solution
1. Take the initial buret reading and record.
Reading the buret. A buret is read at the bottom of the meniscus at eye level. In a
buret, the zero mark is at the top and the number markings count up going down towards
the tip, as shown in Figure 2a. (Keep in mind that this is opposite to the graduated
cylinder where zero is at the bottom.) For example, the liquid level in Figure 1a is
between 1 and 2 mL, but closer to 2 mL; the buret reading is 1.88 mL. For practice, read
the buret in Figure 1b.
(a)
(b)
reading = 1.88 mL
reading = ________
V of titrant used = __________
Figure 1. How to read a buret
The readings are not actual volumes. Buret readings are taken before and after dispensing
the titrant during a titration analysis, so the volume of titrant used is the difference
between the readings. If Figures 1a and 1b show liquid levels at the beginning and at the
end of a titration analysis, respectively, how many mL of the titrant is used? Ask your
instructor to confirm your reading for Figure 1b and the volume of titrant dispensed.
2. Place the heat/stir plate with the flask containing the KHP solution below the buret as
shown in Figure 2. Make sure the solution is being stirred. Stirring continues throughout
the titration.
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buret clamp
buret with
NaOH solution
iron stand
125-mL Erlenmeyer flask
with acid solution,
indicator and stir bar
stir plate
Figure 2. Titration Set-up
5. Open the stopcock to start delivering NaOH to the flask. Slow the flow when the pink
color persists longer. Add the NaOH dropwise nearing the endpoint, making sure the
pink color disappears before adding the next drop.
6. When a drop of NaOH changes the solution from colorless to pink and stays pink with
stirring, the endpoint is reached. Record the final buret reading.
7. Perform two additional trials, using two more KHP solutions. Refill the buret with
NaOH solution when necessary.
Note: Do not allow the liquid level to go below the calibrations at the bottom of the
buret. If this is about to occur, record a final reading. Refill the buret, record another
initial reading and then another final reading when titration is completed. The total
volume of NaOH used is the sum of the volumes used for the first and second sets of
readings.
8. Rinse the buret with water when all titrations are completed.
9. Calculate the molarity of the NaOH solution. Check the values obtained in each trial
with your instructor before proceeding with the vinegar titration. The average NaOH
molarity will be used as the known concentration of the standard solution in the titration
of vinegar.
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II. Analysis of Vinegar
1. Prepare the buret for titration.
a. If continuing the titration on the same day: Refill the buret with NaOH solution.
b. If continuing the titration on another day: Prepare the buret using the NaOH
solution as the titrant, as outlined in Part IB.
2. Record the initial buret reading.
3. Obtain ~50 mL vinegar in a clean beaker. Condition a 10.00-mL pipet with the vinegar
by drawing vinegar above the mark in the pipet then draining it into the waste beaker.
(For a complete review of pipet use, refer to the experiment “Density”.)
4. Using the pipet, transfer 10.00 mL of vinegar into a clean 125-mL Erlenmeyer flask.
Add ~25 mL distilled water into the flask. Add one drop of phenolphthalein. Stir using
the magnetic stirring plate.
Note: Again, the volume of the water added to the flask does not need to be known
accurately. The calculation will be based on 10.00-mL volume of vinegar being analyzed.
5. Titrate. Record the final buret reading.
6. Perform two more trials.
7. Calculate the acetic acid concentration of vinegar in molarity for each trial. Use the
concentration of NaOH solution calculated in Part I.
8. Convert each molar concentration to mass percent. Assume the density of vinegar is
1.01 g/mL. Report the average mass percent.
CLEAN-UP




Dispose of wastes in the large jug in the front hood.
Wash all glassware used. Rinse the pipet.
Rinse the buret with water and drain by clamping it upside down.
Return materials where they belong. Return the stir bar on top of the heat/stir plate.
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Name: ____________________________________
Date: _________________
Partner’s Name: ____________________________
TITRATION: ANALYSIS OF VINEGAR
Report all your measurements and answers in the correct number of significant figures and units.
I. Standardization of the NaOH solution
Trial 1
Trial 2
Trial 3
mass of KHP
initial buret reading (NaOH)
final buret reading (NaOH)
volume of NaOH solution
molarity of NaOH solution
average molarity
Calculate the molarity of the standard NaOH solution for the three trials and report the results
the table above. Show the complete calculations for Trial 1 only. Observe correct number of
significant figures and units.
Trial 1 calculation for molarity of the standard NaOH solution:
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II. Analysis of vinegar
Average molarity of standard NaOH solution from Part I = ____________________
Trial 1
Trial 2
Trial 3
volume of vinegar
initial buret reading (NaOH)
final buret reading (NaOH)
volume of NaOH solution
molarity of HC2H3O2
mass % of HC2H3O2
average mass %
Calculate the molarity and mass percent of acetic acid in vinegar for the three trials and report
the results and the average mass % in the table below. Show the complete calculations for Trial
1 only. Observe correct number of significant figures and units.
Trial 1 calculation for molarity of HC2H3O2 in vinegar:
Trial 1 calculation for mass % of HC2H3O2 in vinegar:
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POST-LAB EXERCISES
Show clearly the complete calculations with correct number of significant figures and units.
1. A solution of nitric acid is standardized using 0.851 g of sodium carbonate. Find the
molarity of the nitric acid solution if 17.66 mL of it are required to reach the endpoint.
2HNO3(aq) + Na2CO3(aq)  2NaNO3(aq) + H2O(l) + CO2(g)
2. Caltrate tablets contains calcium carbonate. If one tablet requires 34.46 mL of 0.291 M
solution of hydrochloric acid for neutralization, how many milligrams of calcium carbonate
is in this Caltrate tablet?
CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g)
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3. In the titration analysis, a 10.0-mL sample of household ammonia solution (density = 0.990
g/mL) required 25.16 mL of a 0.231 M HCl solution.
NH3(aq) + HCl(aq)  NH4Cl(aq)
(a) Calculate the molarity of this ammonia solution.
(b) Calculate the mass % of ammonia in the solution.
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