write a lab chemistry report for TITRATION OF THE ACETIC

EXPERIMENT #6: TITRATION OF THE ACETICACID IN VINEGAR
Purpose:
The purpose of this experiment is to determine the amount of acetic acid in vinegar by
titrating it with a standard (known concentration) solution of NaOH in the presence of
the acid-base indicator phenolphthalein.
Special Apparatus and Chemicals:
Standard NaOH Solution
10 mL Volumetric Pipet
50 mL Buret
Dropper bottle with phenolphthalein
5% white vinegar
Buret Clamp and Stand
Discussion:
The concentration of a solution is often determined by volumetric analysis in which a
given volume of the solution, measured with volumetric glassware, is analyzed
against a knownsubstance or solution. In our experiment we will start with a known
volume of the vinegar whoseconcentration is to be determined, and amounts of a
standard solution of sodium hydroxide of known concentration will be added
incrementally until the reaction (neutralization) is complete. This process of
incremental addition until a complete reaction is achieved is called titration. The
standard solution is referred to as the titrant and the substance being analyzed the
analyte. The volume of the titrant needed for neutralization is determined by using a
buret. Before beginning this experiment be sure to read about using a buret beginning
on page 31 of this manual.
In this experiment you will be determining the concentration of a vinegar solution
(dilute acetic acid) using a standardized solution of sodium hydroxide. Because this and
most other acid-base neutralizations occur with no visible reaction, an acid-base
indicator such as phenolphthalein is used to give a visible sign that neutralization has
occurred. Phenolphthalein is colorless in acidic or neutral solutions, but pink in basic
solutions. Thus, by putting phenolphthalein in our sample of vinegar that is being
titrated, the initial solution is clear but becomes pink upon neutralization of the acetic
acid and addition of excess sodium hydroxide. Thestage at which the vinegar has been
neutralized and one extra drop of titrant causes the indicator to change colors is called
the end point. In very precise work, the sodium hydroxide itself would be standardized
against a primary standard such as potassium hydrogen phthalate.
Initially you will know the concentration of the standard sodium hydroxide solution
and the volume of vinegar that you are titrating. After carrying out the titration you
will also know the volume of NaOH that is needed to neutralize your given volume of
vinegar. Using this information, you can easily calculate the concentration of acetic
acid in your vinegar. Because the reaction of acetic acid and sodium hydroxide occurs
in a one to one molar relationship,
HC2H3O2 + NaOH –> NaC2H3O2 + H20
the following simple relationship holds true when concentration is in terms of
molarity:
(Concentration HC2H3O2) = (Concentration NaOH X Volume NaOH/Volume
HC2H3O2)
This simple relationship works because: the concentration in molarity (moles/liter)
multiplied by a volume gives the number of moles for a compound, and the number of
moles of HC2H3O2 must be equal to the number of moles of NaOH at neutralization.
CAUTION: This particular relationship only works for reactions with a one-toone stoichiometry; otherwise, you should work out the concentration of unknown
using a complete dimensional analysis procedure.
For example: If 34.50 mL of 0.2486 M NaOH is needed to titrate 10.00 mL of a HCl
solution, what is the concentration of the HCl?
HCl + NaOH ——> NaCl + H2O
Concentration HCl = 0.2486M NaOH x 34.5OmL NaOH/l0.00 mL HCl = 0.8577M
HCl
The stoichiometry of NaOH to HCl is 1:1; therefore, we can use our simple
relationship, and the concentration of the HCl solution is therefore 0.8577 M.
Concept:
You will determine the concentration of acetic acid in vinegar by titrating a sample of
vinegar with a known solution of sodium hydroxide. Phenolphthalein will be used as
an indicator dye to determine when the reaction is complete. In basic solutions
phenolphthalein is pink, in acidic solutions it is clear. NaOH will be added to a known
volume of vinegar containing a few drops of phenolphthalein. As the neutralization
reaction is complete and the next drop of NaOH is added, the solution will go from
acidic to basic and change color from clear to very light pink signaling the end-point
of the titration. Knowing the volumes of vinegar and sodium hydroxide used and the
concentration of the standard sodium hydroxide solution, the acetic acid concentration
in the vinegar can be determined.
Procedure:
Clean and prepare a buret according to the directions given in the section on
“Measurement of Volume” in the beginning of this manual. Fill the buret with NaOH.
Make sure no air bubbles are left in the tip of the buret. Be sure to write down the
concentration of the NaOH and the initial buret reading. Read the buret to within
±0.02 mL; remember to read from the bottom of the meniscus. CAUTION: NaOH
may cause chemical burns. If any NaOH comes in contact with your skin or clothes,
rinse the affected area immediately with large amounts of cold water.
Pipet exactly 10 mL of white distilled vinegar into a clean 125 mL Erlenmeyer flask
using a volumetric pipet. Note: Pipetting should never be done out of the original
reagent bottle; in order to avoid contamination, transfer some of the liquid being
pipetted to a small beaker or flask before pipetting. Add 10 mL of distilled water and
2 or 3 drops of phenolphthalein solution to the vinegar. You are now ready to begin
the titration. (Note: It is often helpful to do an initial trial titration to find the
approximate volume of titrant needed. Check with your instructor about doing a trial
titration.)
Begin the titration by letting a volume of sodium hydroxide run into the flask equal to
the volume that you think is needed for neutralization minus 1 to 2 mL. You should
now be within 20-40 drops of the end point (1 mL equal about 20 drops). Swirl the
flask to mix the solution and then rinse down the walls of the flask with distilled water
from a squirt bottle. Now add the sodium hydroxide drop by drop until the end point
is reached. The sodium hydroxide should be added while maintaining a gentle
swirling motion of the flask. The solution should be a very light shade of pink when
the titration is finished. A piece of plain white paper placed under the flask is often
helpful in determining when the end point is reached. Record the final buret reading.
If you are unsure whether you have reached the end point, record the reading and then
add one more drop. If you were at the end point, the extra drop will give the solution a
very definite pink color.
DISPOSAL: All solutions from this experiment may be safely disposed of down the
sink with running water.
Refill the buret with NaOH and rinse out the flask with distilled water. Repeat the
titration three more times or as directed by your laboratory instructor. After you have
collected a good set of data and with due regard for significant figures, calculate the
acetic acid concentration in the vinegar for each trial; determine the average
concentration of acetic acid using only the measurements that you are confident
in (i.e. those in which you did not overshoot the endpoint); and determine the standard
deviation for your concentration values.
REPORT SHEET: TITRATION OF ACETIC ACID IN VINEGAR
Name: ____________________________________
Section: ___________________________________
Date:______________________________________
Concentration of the NaOH solution _____40______ M
Volume of NaOH used during trial titration _____21.0_______ mL
TITRATIONS
trails #1
#2
#3
#4
Final buret reading
___21.4___ __45____ __65____ ____87__mL
Initial buret reading
___0___ ___21.0___ ___45___ ____65__mL
Volume of NaOH solution
____21.0__ ___24___ ___20___ ___22___mL
Concentration of HC2H3O2 (show calculations)
______ ______ ______ ______M
Calculate the concentration of HC2H3O2
And do the average concentration and the stander deviation
Average concentration ___________________
Standard deviation __________________________
Expriment 6
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