Titration of Vinegar Lab
PRELAB
1. Write and balance the reaction that will occur in this lab. Do NOT abbreviate the acetate
ion. Show phases.
2. Reduce the reaction in #1 to the net ionic equation. Include charges and phases.
3. Calculate the number of grams of NaOH (s) needed to make 100.0 ml of 0.10 M NaOH.
Show work and answer in prelab and also in your data table for procedure part I.
4. What happens at the equivalence point? At the endpoint? Which one happens first?
5. Will the solution be acidic, neutral, or basic at the equivalence point. Explain your choice.
6. Read the procedure section and label the following
schematic diagram with the names of each piece of
equipment and the contents of all the containers.
PURPOSE
Determine the concentration of acetic acid in vinegar, in terms of % by mass and molarity.
THEORY
Neutralization is a reaction of an acid with a base to produce a salt and water. For example, the
reaction of the strong acid HCl with the strong base NaOH produces the salt sodium chloride
and water:
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
Since the acid, base, and salt in this reaction are all strong electrolytes, the net ionic equation
for this reaction is:
H+(aq) + OH-(aq)  H2O(l)
This same result occurs in the reaction of strong acid reacts with a weak base, or a weak acid
reacts with a strong base. The acid and base react to form a salt and water. The neutralization
reaction provides us with one method for determining the amount of either the acid or the base
in a solution.
The general method employed is that of titration. Titration is a procedure for determining the
amount of substance A by adding a carefully measured volume of a solution with known
concentration of B until the reaction between them is just complete. Titrations are not use
exclusively with acids and bases but anytime you need to determine the concentration of a
species in a solution.
In this lab, we will titrate a vinegar solution with a solution of sodium hydroxide, NaOH. The
active ingredient in vinegar is the weak acid, acetic acid. The structure is draw below, the red
hydrogen is the acid hydrogen. Meaning it will be the only hydrogen involved is this acid-base
reaction. The abbreviation for acetic acid is HAc.
H O
H C C O H
H
acetic acid
The sodium hydroxide will neutralize the acetic acid, HAc, in the vinegar. The Ac actually
represents the acetate anion, C2H3O2-.
HAc(aq) + NaOH(aq)  NaAc(aq) + H2O(l)
Once the neutralization reaction is complete, any additional base added will produce a basic
solution. The point at which all the acetic acid has reacted with the sodium hydroxide is called
the equivalence point. We can determine when the titration is complete by employing an acidbase indicator which will change color in a basic solution. Phenolphthalein is an acid-base
indicator that changes color from colorless to magenta. This particular indicator turns from clear
to pink in basic solution. If one extra drop of base is added after the acid has been neutralized
the pH will jump significantly. Phenolphthalein changes color at pH=8.2, one extra drop of base
will cause the solution to change color. So, technically, the endpoint (color change) is one drop
beyond the equivalence point.
O-
HO
OH
2 OH
O
2 H2O
CO2-
O
O
As soon as very faint magenta color persists you have reached the equivalence point. At the
equivalence point, the number of moles acid will equal the number of moles base, hence:
Molarityacid x Volumeacid = Molaritybase x Volumebase
Thus, we can determine the molarity of the acid by titrating a known volume of the acid solution
with a known concentration and volume of a base.
Before we can proceed with the titration of the acid, we must first determine very accurately the
concentration of the sodium hydroxide solution. Although sodium hydroxide is a solid,
preparation of solutions of accurately known concentration is difficult. The base is very
hygroscopic, water absorbing. Additionally, the resulting solution tends to absorb carbon
dioxide from the air, which neutralizes some of the base. Hence, we first standardize the base
against an appropriate primary standard. Titration standards must be stable and be capable of
being measured for amount very accurately. A typical standard for bases is the weak acid
potassium hydrogen phthalate, KHC8H4O4, which is frequently abbreviated KHP. This reaction
is in a one mole ratio:
KHP(aq) + NaOH(aq)  KNaP(aq) + H2O(l)
By knowing the masses of KHP, we can determine the number of moles used in the titration:
mass KHP
moles of KHP 
molar mass of KHP
At the equivalence point, then:
moles NaOH = moles KHP
Thus, the molarity of the NaOH solution is determined via the volume of the base used in the
titration:
moles base
Molarity of base 
volume base
PROCEDURE
Part 1: Preparation of approx. 0.1M NaOH
1. Prepare 100 mL of an approximately 0.1M solution of NaOH by dissolving the appropriate
amount of NaOH in water in a 100 ml volumetric flask.
a. Add deionized water about 2/3 of the way up the volumetric flask.
b. Quickly weigh the solid NaOH and add it to the volumetric flask (make quick
observations of the NaOH as you are doing this step and think about how this is
affecting your results.
c.
d.
e.
f.
Swirl vigorously to dissolve the NaOH.
Add deionized water to the mark on the flask (read the bottom of the meniscus).
Stopper the flask, invert to mix.
Recalculate the Molarity if you used more or less than the original grams of NaOH
calculated.
Part 2: Preparation of the buret
1. Obtain a buret, make sure the valve moves and is tightly secured to the bottom of the
buret.
2. Pour approximately 25 ml of soapy water into the buret and tip and roll the buret with the
open end over a sink to coat all the walls of the buret. Allow some of the soapy water to
run through the buret tip to clean it. Pour excess soapy water out of open end of the
buret.
3. Rinse the buret and valve a few times with tap water until no more bubbles of soap are
evident.
4. Rinse the buret with deionized water.
5. Finally, pour approximately 2 ml of the titrant into the buret. Roll the buret with the open
end over a sink until the titrant coats the walls of the buret. Allow some of the titrant to
flow through the buret tip. Pour the excess titrant out of the open end of the buret.
6. Now, affix the buret to the buret clamp, make sure it is over a sink and that the top of the
buret is below eye level. Fill the buret to the zero mark with titrant. Allow some titrant to
flow through the buret tip, then refill to zero or read the new starting volume to two
decimal places.
7. Perform the titration of vinegar making sure to never allow the titrant to go below the
50.00 ml mark on the buret.
Titration of Vinegar
1. Weigh a clean Erlenmeyer flask.
2. Use a volumetric pipet to transfer 5.00 mL of vinegar to a 250 mL Erlenmeyer flask. Weigh
again.
3. Dilute this to about 50 mL with deionized water.
4. Add a few drops Phenolphthalein. Note the color of the solution.
5. Titrate to the equivalence point with your NaOH solution.
a. Do not allow the NaOH to drain below the 50.00 ml mark on the buret.
b. Read initial and final volumes to 2 decimal places.
c. Add titrant slowly while stirring constantly until the phenolphthalein pink starts to be
seen in solution. Rinse the sides of the Erlenmeyer frequently to be sure that the all
of the titrant mixes into the analyte. When the pink color starts to persist, add the
titrant drop-wise with continuous stirring until the color remains pink and cannot be
swirled out.
d. Note the final volume of base added and calculate the total volume delivered.
6. Repeat this process for a total of three trials.
DATA
PART I
Mass of NaOH needed
Show calculation:
Qualitative observations while making the solution. Observe crystals of NaOH and note whether
the combined solution volume was exactly on the meniscus or above or below.
PART II
Trial #1
Volume of vinegar
Mass of vinegar
Density of vinegar
End volume of base
Beginning volume of
base
Total volume of base
delivered
Moles of base
delivered
Moles of acetic acid
reacted
Grams of acetic acid
reacted
Molarity of vinegar
% by mass of acetic
acid in vinegar
Theoretical % by
mass of acetic acid
% error
Trial #2
Trial #3
Average
CALCULATIONS
Show work, units and sig figs for the following calculations.
Calculate the density of vinegar.
Using your density of vinegar, determine the mass of vinegar used.
Determine the number of moles of base delivered.
Determine the number of moles of acetic acid in the vinegar.
Determine the molarity of acetic acid in vinegar.
Determine the grams of acetic acid in the vinegar and use this number of grams/ grams total
solution * 100 to find the mass %.
Compare this mass % with the label claims in a % error calculation.
POST LAB
1. What volume of each of the following acids will react completely (equivalence point) with
50.00 ml of 0.200M NaOH? (6 point problem)
a. 0.100 M HCl
b. 0.0500 M H2SO4
c. 0.250 M HC2H3O2
2. Calculate the initial pH of the solutions in 1 a, b, and c. Show all work. Does the initial
pH of the acid have any bearing on the overall volume of acid needed to reach
equivalence? Support your claim. (12 point problem)
3. A student titrates an unknown amount of potassium hydrogen phthalate (KHC 8H4O4
abbreviated KHP), with 20.46 ml of a 0.100 M NaOH solution. KHP (molar mass =
204.22 g/mol) has one acidic hydrogen. How many grams of KHP were titrated (reacted
completely) by the sodium hydroxide solution. (5 point problem)
4. Carminic acid, a naturally occurring red pigment extracted from the cochineal insect,
contains only carbon, hydrogen, and oxygen. It was commonly used as a dye in the first
half of the nineteenth century. It is 53.66% C and 4.09% H by mass. A titration required
18.02 ml of 0.0406 M NaOH to neutralize 0.3692 g of the monoprotic acid. What is the
molecular formula of the acid? (10 point problem)