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)
acid
+
base
OH-1 (aq)
water

H2O(l)
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 color
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. Because the H+1 and OH-1 react in a I to 1
ratio, you will know the number of moles in the unknown.
The first part of this lab deals with the standardization of a solution of sodium hydroxide. Sodium
hydroxide, NaOH, is a very reactive base. Solid or aqueous sodium hydroxide reacts with carbon
dioxide from the air. The solid form also absorbs water from the air. In measuring the mass of NaOH, it
is exposed to air, reacts and therefore is not pure. In dissolving this sample of sodium hydroxide, we do
not know the exact concentration. The concentration of the sodium hydroxide solution is found by
titration against a standard.
The standard acid solution is produced using potassium hydrogen phthalate (KHP). KHP is a very stable
acid (does not decompose or react with air) and is monoprotic (produces 1 mole of hydrogen ions per
mole of acid).
KOOCC6H4COOH(s) ↔
potassium hydrogen
phthalate
K+1(aq) + H+1(aq) + OOCC6H4COO+2(aq)
potassium
hydrogen
phthalate
In Part II of this lab, the standardized sodium hydroxide solution will be used to titrate the acid in
pain-killers - ASA (acetylsalicylic acid). ASA is a monoprotic acid:
CH3COOC6H4COOH(s)
↔
CH3COOC6H4COO-1(aq)
+
H+1(aq)
(Do not let the formula scare you; we are not going to be concerned with the structure at this time.)
PROCEDURE
PART I
Rinse the buret with 5-10 mL of the NaOH solution. Fill the buret body with the sodium hydroxide
solution. Run a small amount of the solution through the tip to ensure that the tip is filled completely.
Make sure that there are no air bubbles trapped in the tip.
Obtain 20.0 mL of 0.100M KHP solution and place the solution into a 250 mL Erlenmeyer flask. Add a
couple of drops of the phenolphthalein indicator solution to this flask.
Record the initial volume of the NaOH in the buret(± .01 mL). Gently swirl the flask as you add the
NaOH solution. As you approach the end point (neutralization), the pink color persists for longer periods
of time and you should start adding the sodium hydroxide slowly (by drops). During the titration, rinse
the interior walls of the flask with distilled water. This is done to mix the acid with all of the base added.
The end point of the titration occurs when the solution in the flask turns a very faint pink. The pink color
should persist for at least 30 seconds. Record the final volume of the NaOH in the buret.
Repeat the titration until you get consistent results.
PART II
Obtain the mass of 5 pain-killing tablets as directed by your teacher. Crush one tablet. Transfer the
crushed tablet to a 250 mL Erlenmeyer flask and add 25 mL of ethanol. Swirl the contents of the flask to
allow the mixing of the tablet and ethanol. The tablet filler will not dissolve but the ASA from the tablet
will dissolve. Add a couple of drops of phenolphthalein to the flask. Titrate the ASA with the
standardized sodium hydroxide solution.
Repeat Part II until you get consistent results.
MAKING SENSE OF THE DATA
1. Using the consistent trials, complete the following table. Show sample calculations.
Concentration of KHP(aq)
2.
Volume of KHP(aq)
Volume of NaOH(aq) added
Concentration of NaOH(aq)
Calculate the average mass of a tablet.
3. Using the consistent trials, complete the following table. Show sample calculations.
Concentration of NaOH(aq)
Volume of NaOH(aq) added
Moles of OH-1(aq) added
Moles of ASA present
4.
Calculate the percentage of the tablet which is ASA.
5.
Calculate the mass of NaOH(s) required to produce 250.0 mL of 0.10M NaOH(aq)