Experiment V02
Chemicals:
An analysis of aspirin tablets
Standard 1.0 M NaOH, (25 cm3)
standard 0.050 M H2SO4, (100 cm3)
aspirin tablets, (1.5 g, 2-3 tablets)
phenol red indicator (or phenolphthalein indicator),
litmus indicator.
Apparatus:
burette,
25 cm3 pipette,
250 cm3 conical flask,
250 cm3 volumetric flask,
white tile,
glass rod,
tripod,
gauze,
Bunsen burner,
clamp and stand,
washing bottle.
Introduction:
Aspirin (a compound of ethanoic acid and salicylic acid) can be readily hydrolysed by sodium
hydroxide into the sodium salts of two weak acids, acetic acid and salicylic acid. In this
experiment, the hydrolysis is effected using an excess of sodium hydroxide, the excess sodium
hydroxide being later found by titration with standard sulphuric acid.
The equation for the hydrolysis reaction is:
CH3COOC6H4COOH(s) + 2NaOH(aq)  CH3COONa(aq)+ HOC6H4COONa(aq)+ H2O(l)
Aspirin
The presence of the weak acid salts (hydrolysis products) means that the resulting solution is best
titrated using phenol red indicator (pH range 6.8 to 8.4), although phenolphthalein is satisfactory.
In this experiment you will have the opportunity to carry out a consumer survey on the aspirin
content of a number of commercial preparations, to see whether the manufacturer's claims are
justified.
P.1
Experiment V02
An analysis of aspirin tablets
Procedure
a.
Weigh accurately a definite number of aspirin tablets (two or three tablets weighing no
more than 1.5 g) into a 250 cm3 conical flask. Initiate the hydrolysis of the aspirin by
adding 25 cm3 of 1.0 M sodium hydroxide by pipette, diluting with approximately the
same volume (25 cm3) of distilled water. Warm the flask over a tripod and gauze for ten
minutes to complete the hydrolysis.
b.
Cool the reaction mixture and transfer with washings to a 250 cm3 volumetric flask,
dilute to the mark with distilled water and then ensure that the contents of the flask are
well mixed by repeated shakings.
c.
Titrate 25 cm3 portions of the diluted reaction mixture with the standard 0.050 M
sulphuric acid provided, using phenol red indicator until two or three consistent results
are obtained.
d.
Calculate the weight of acetylsalicylic acid in each tablet and compare your results with
the manufacturer's specification. Compare your results with those given in different
brands of aspirin.
Titration Results
Titration
trial
1
2
3
Final burette reading (cm3)
Initial burette reading (cm3)
Volume of acid used (cm3)
Average volume of H2SO4 used =
Questions
1.
Explain why it is necessary to warm the reaction mixture of aspirin and sodium hydroxide
solution.
2.
Explain why phenol red (or phenolphthalein) is used as the indicator in this experiment.
P.2
Experiment V02
An analysis of aspirin tablets
Name:
Seat No.:
Date:
Grade:
Brand
Given: [H2SO4] =
[NaOH] =
Calculation:
Mass of aspirin =
Average vol. of H2SO4 used (in procedure c) =
No. of moles of H2SO4 used =
=
H2SO4 + 2NaOH  Na2SO4 + 2H2O
No. of moles of excess NaOH in the reaction mixture (25 cm3, in conical flask) =
No. of moles of excess NaOH in the volumetric flask (250 cm3) =
=
=
No. of moles of NaOH in the original solution (procedure a) =
=
No. of mole of NaOH reacted with aspirin =
=
CH3COOC6H4COOH(s) + 2NaOH(aq)  CH3COONa(aq) + HOC6H4COONa(aq) + H2O(l)
No. of moles of acetylsalicyclic acid =
=
Mass of acetylsalicyclic acid =
=
Mass of acetylsalicyclic acid in each tablet =
=
% by mass of acetylsalicyclic acid in the tablet
=
=
P.3