ACADEMIC CHEMISTRY
PERIOD
NAME:
DATE:
ASPIRIN TITRATION LAB
PRELAB
Read the entire lab and answer the following questions. You may need to consult the internet to
answer some of the following questions.
1.
2.
3.
4.
What is the name of the acid included in aspirin?
What is the formula of aspirin?
What is aspirin’s theoretical molar mass?
Draw a structural formula for acetylsalicylic acid and label the carboxyl group and the acidic
hydrogen.
PURPOSE
1. conduct an acid base titration to determine the molar mass of aspirin
2. measure the volume of a known concentration of basic solution using proper titration techniques
and equipment (buret)
3. calculate the number of moles of base needed to reach the endpoint and compare to the number
of moles of acid
BACKGROUND
Aspirin is slightly acidic and reacts with bases in neutralization reactions. If the reaction is
followed with an indicator, a color change will occur when the acid is completely neutralized and one
drop of excess base has been added. The number of moles of base consumed and the number of
moles of acid in the sample can be calculated form the volume of the base needed to obtain the color
change. The relationship between these two mole values is determined form the balanced chemical
equation for the reaction.
In this experiment, you will titrate an aspirin tablet with a known concentration of sodium
hydroxide solution. The end point will be determined with phenolphthalein or thymol blue indicator.
The following simplified equation describes the neutralization reaction that will be observed in this
experiment.
H-Asp (aq) + NaOH (aq)  Na-Asp (aq)
+ H2O (l)
Because the mole ratio is 1:1, the number of moles of the base will equal the number of moles of acid
at the equivalence point of the titration.
The molar mass of the solid aspirin can be approximated by dividing the mass of the aspirin in
the tablet by the moles of aspirin present. The mass of the aspirin inside the tablet is usually near
325 mg. Inactive ingredients, such as binders, are added to the aspirin during the manufacturing
process. Therefore, the actual mass of the tablet exceeds 325 mg because it is not 100% pure
aspirin.
MATERIALS
100 ml of distilled water in a wash bottle
50 ml of 0.100 M NaOH
40 mL of 95% ethanol
phenolphthalein or thymol blue indicator (3 drops per titration)
325 mg non buffered aspirin tablets
1000 ml graduated cylinder
250 ml Erlenmeyer flasks, 2
ACADEMIC CHEMISTRY
PERIOD
buret
buret clamp and ring stand
funnel
NAME:
DATE:
PROCEDURE
Please wear goggles throughout the lab. Thoroughly wash away any solutions that you may get on
your skin.
1.
2.
3.
4.
PARTNER A
Assemble the ring stand, buret clamp, and buret.
Clean the buret according to the procedure outline in class.
Rinse the buret with 3 mL of 0.100 M NaOH solution.
Fill the buret with 0.100 M NaOH to just above the 0.00 mark. Allow some of the solution to
drain through the tip and record the initial volume of NaOH to 2 decimal places.
PARTNER B
1. Weigh an empty Erlenmeyer flask. Add an aspirin table to the flask and weigh again. Add
about 20 ml of distilled H2O. Wait 20-30 seconds. The tablet will break apart as it absorbs
water and swells. Then add about 20 ml of ethyl alcohol to help dissolve the aspirin.
2. Add 3-4 drops of phenolphthalein indicator and swirl the flask.
TOGETHER
1. Titrate to the endpoint with 0.100 M NaOH. Record in the data table the volume of base that
was required.
2. Repeat the procedure with a second aspirin tablet. IF time permits, you may perform a 3 rd
titration.
3. Wash all equipment when finished. Be sure to rinse the buret with distilled water and leave it
to dry by inverting it in the buret clamp.
ACADEMIC CHEMISTRY
PERIOD
DATA
TRIAL
Mass
of NaOH
asprin
initial
tablet
volume
reading
NAME:
DATE:
NaOH final Volume of Volume
Concentration of NaOH
volume
NaOH
of NaOH
reading
delivered
delivered
(L)
Units:
1
2
3
TRIAL
Moles
NaOH
endpoint
of Moles
at aspirin
endpoint
Units:
1
2
3
CALCULATIONS
1. Volume of NaOH delivered
2. Volume of NaOH delivered (L)
3. Moles of NaOH at endpoint
4. Moles of aspirin at endpoint
of (experimental) (theoretical)
% error
at Molar mass of Molar mass
aspirin
of aspirin
ACADEMIC CHEMISTRY
PERIOD
NAME:
DATE:
5. Experimental molar mass of aspirin
6. % aspirin in the tablet
7. Average molar mass of aspirin
8. Average deviation of the three trials (experimental molar masses)
9. Average % error
POST LAB
1. If there was another additive to the aspirin tablet, say vitamin C, how would this affect the
results for the titration of the aspirin?
2. If titrated properly, the neutralized aspirin solution color will fade over time. Why might this be?
(hint: relates to CO2 in the air).
3. An impure sample of ascorbic acid, with a mass of 2.06 grams is dissolved in water. It is
titrated with 0.200 M sodium hydroxide. The sample required 46.0 ml to reach an endpoint
using phenolphthalein as the indicator.
a. Calculate the number of moles of ascorbic acid in the sample.
b. Calculate the mass of ascorbic acid in the sample (hint: you need the molar mass of
ascorbic acid)
c. Calculate the % purity or the sample (mass of ascorbic acid/ mass of sample X 100)
4. Calculate the concentration of acetic acid, if an average of 67.0 ml of .468 M magnesium
hydroxide is required to titrate 50.0 ml of the acid in order to reach the endpoint.
a. Write the balanced neutralization reaction that occurs.
b. Calculate the concentration of acetic acid.
5. Calculate the concentration of KOH if 65.0 ml of the base are used to titrate a 15.0ml sample
of 1.0M H2SO4.
a. Write a balanced neutralization reaction.