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Chemistry 12
Analysis of Pain Killers (ASA)
Introduction
Titration is a quantitative analytical technique. Acid-base titrations are based on reactions between
acids and bases. Acid-base reactions can be represented by the following equation:
H+1 (aq) + OH-1 (aq)  H2O (l)
acid
base
water
The equation above shows one mole of hydrogen ions, from the acid, reacting with one mole of
hydroxide ions, from the base, to form one mole of water. Acidic and basic properties are neutralized
in the formation of water.
The solution of a stable compound of known concentration is a standard solution. All unknown
concentrations are calculated relative to the standard solution. Titration is the process of mixing the
standard and unknown solutions. An indicator present during the mixing of solutions changes colour
when neutralization occurs. Knowing the volume and the concentration of the standard solution
allows you to calculate the number of moles of H+1 or OH-1 added. Since the H+1 and OH-1 react in a
1 to 1 ratio (at least for this experiment…this IS NOT always the case), you will know the number of
moles in the unknown.
In this lab, the standardized sodium hydroxide solution will be used to titrate the acid in pain-killers ASA (acetylsalicylic acid). ASA is a weak monoprotic acid:
CH3COOC6H4COOH (s)
CH3COOC6H4COO-1 (aq) + H+1 (aq)
Procedure
Obtain 50 mL of standardized NaOH solution and prepare the burette for titration. Obtain the mass of
five (5) pain-killing tablets as directed by your teacher. Crush one tablet. Transfer the crushed tablet
to a 250 mL Erlenmeyer flask and add 10 mL of ethanol. Swirl the contents of the flask to allow the
mixing of the tablet and ethanol. Note, the tablet filler will not dissolve in the ethanol however the
ASA from the tablet will dissolve. Add a one (1) or two (2) drops of phenolphthalein to the flask.
Titrate the ASA solution with the standardized sodium hydroxide solution to its equivalence point.
Repeat the above procedure until you get consistent results.
Making Sense of the Data
1. Calculate the average mass of a tablet.
2. Using the consistent trials, complete the following table. Show one set of sample calculations.
Concentration of
NaOH (aq)
Volume of NaOH (aq)
added
Moles of OH-1 (aq)
added
Moles of ASA
present
3. Calculate the percentage of the tablet which is ASA.
4. Calculate the mass of NaOH (s) required to produce 250.0 mL of 0.10M NaOH (aq)