Titration of Aspirin
A titration is a lab process where substances are combined using volumetric glassware,
such as buret, in a carefully controlled way such that the exact amounts needed to react are used.
You already know from the Bonding Lab, that aspirin is weak electrolyte. Aspirin is a
weak electrolyte because it is weakly acidic. Commercially available aspirin tablets have aspirin
and an inert binder to hold the tablet together. Your task will be to determine how much aspirin is
in the tablet and how much of the inert binder is in the tablet. Aspirin (acetyl salicylic acid
C9H8O4, abbreviated HAS) reacts with sodium hydroxide according to the following equation:
HAS(aq) + NaOH(aq)  H2O(ℓ) + NaAS(aq)
In the Lab:
1. Burets are probably the single most expensive piece of equipment in the lab. Be
exceedingly careful with the burets.
2. One of the difficulties associated with NaOH is that it tends to react with atmospheric
carbon dioxide: NaOH + CO2  NaHCO3. This presents a problem because the amount
of NaHCO3 will cause a concentration calculated strictly from the mass of NaOH used to
make the solution to be higher than the actual concentration. To account for how much
sodium hydroxide is actually dissolved in solution, Mrs. Atkins will titrate the NaOH
solution with a primary standard, potassium hydrogen phthalate, KHC8H4O4 (abbreviated
KHP). You will use the information provided from this standardization to calculate the
concentration of NaOH solution you used. NaOH reacts with the KHP according to the
following equation: NaOH + KHC8H4O4  H2O + NaKC8H4O4
3. Your buret will have been properly prepared by Mrs. Atkins. Fill the buret with NaOH
solution. It is a waste of time to try to get the liquid level exactly on the 0.00mL line. You
only need to record the exact position of the liquid line and you can determine the volume
added by difference. Burets can be accurately read to 2 decimal places at the bottom of
the meniscus.
4. Find the mass of an aspirin tablet using the electronic balances. Place this aspirin tablet in
a 250mL Erlenmeyer flask. Add roughly 20mL of distilled water to the Erlenmeyer flask.
Wait around thirty seconds for the aspirin tablet to swell and fall apart. After the tablet
falls apart, add roughly 20mL of ethanol to dissolve the aspirin. Swirl the solution to
ensure that it is homogenous. There is likely to be undissolved binder remaining in the
flask. This is ok.
5. Add 5 drops of phenolphthalein indicator to the flask. Phenolphthalein turns pink in basic
solutions.
6. Slowly titrate the aspirin with the NaOH. You should vigorously swirl (not shake) the
flask continuously to ensure the solution is homogeneous. The end point is the faintest
possible pink color that you can perceive. Ideally, one drop of NaOH should change your
solution from colorless to bright pink. This is rather rare. You should focus on getting the
lowest possible level of pink you can. Record your final volume of NaOH added.
7. Pour your mixture in the Erlenmeyer flask down the drain with lots of water.
8. Repeat the procedure with a second aspirin tablet. You must get at least two good trials.
This means that you will probably do more than just two trials.
9. Mrs. Atkins will properly clean your buret.
Lab Report:
A) Purpose
B) Analysis Questions
1) Show your calculations for the molarity of the NaOH from each of the
three standardizations.
2) Show your calculation for the average molarity of the NaOH based on
the standardization.
3) Show your calculations for the mass of aspirin that reacted in each of
the two trials based on the amount of NaOH used.
4) Show your calculations for the average mass of aspirin in a tablet of
aspirin.
5) Show your calculations for the percentage of an aspirin tablet that is
aspirin.
6) Aspirin tablets are manufactured to contain 325mg of aspirin.
Calculate your percent error in your mass of aspirin in a tablet:
% Error 
Experimental  Theoretical
Theoretical
 100
7) Suggest two specific errors that would account for your specific error
in your values of aspirin in an aspirin tablet and explain how these
errors would affect your values. You may NOT say generic things like
human error or inaccuracies of the balance. You must instead come up
with specific things that were done in lab that could account for your
values not being exactly 325mg. You must then explain how these
factors affect your results. If you got exactly 325mg on all your trials
then give one reason why someone might have gotten a value above
325mg and one reason for a value below 325mg.
C) Conclusions
D) Original Data Sheet including standardization data