Apparatus
· About 1.5g of aspirin tablets – usually 4 or 5 tablets
· 1.0 mol dm³ Sodium Hydroxide
· 0.1 mol dm³ Hydrochloric Acid
· Distilled Water
· Methyl Orange indicator
· Conical Flask
· Burette
· Burette Stand Acetylsalicylic acid
· Funnel
· Pipette
· Safety Filler
· Bunsen burner
· Bunsen Mat
· Tongs
· 250cm³ Standard Flask
· Goggles
· Labcoat
Apparatus Justification
· Aspirin tablets – These contain the acetylsalicylic acid, which will react with the sodium hydroxide
· Sodium Hydroxide – This will hydrolyse the aspirin into salicylic and acetate ions
· Hydrochloric Acid – This will neutralise the sodium hydroxide
· Distilled Water – This is used to make up the correct solutions and to wash equipment. Normal water will not do because of the impurities in it.
· Methyl Orange indicator – The colour of this indicates when the sodium hydroxide has been neutralised by the hydrochloric acid.
· Conical Flask – This is used to react the aspirin tablets with the sodium hydroxide. It is more appropriate to use as the shape of it makes it less likely that any should spill out.
· Burette – This is used to add the hydrochloric acid to the sodium hydroxide. It makes the results as accurate as possible, as it allows the hydrochloric acid to be added drop by drop when the sodium hydroxide is close to neutralising.
· Burette Stand – This holds the burette steady in place.
· Funnel – This allows hydrochloric acid to be poured into the burette so none is spilt.
· Pipette – This is used to accurately draw out the correct amount of a substance from a solution.
· Safety Filler – This is used to draw the substance up the pipette as safely as possible.
· Bunsen burner – This is used to gently heat up the sodium hydroxide and aspirin solution, increasing the rate at which they react.
· Bunsen Mat – This is used to protect the table
· Wooden splint – Used to light the Bunsen safely
· Matches/Lighter – used to light the wooden splint
· Tripod – Used as a stand to hold the solution in the Bunsen flame
· Gauze – Acts as a fireproof table surface for the flask to sit on
· Tongs – To safely pick up the hot conical flask
· White Tile – So the colour of the solution is easier to see
·250cm³ Standard flask – This is used to mix up solutions accurately.
· Goggles – Used to protect the eyes throughout the experiment
· Laboratory coat – Used to protect the skin and clothing
Safety Precautions
· Sodium Hydroxide is extremely caustic, and may cause severe burns if it comes into contact with the skin. It is particularly dangerous if it gets in the eyes, even in the most dilute solutions, so labcoats and eye protection is essential whilst handling. In order to dispose of, it should first be neutralised using an acid
· Hydrochloric acid is hazardous, so care should be taken not to inhale or swallow, as it can cause severe burns and even kill. It is extremely corrosive to skin and may produce burns and ulcerations. If it gets into the eyes, it may cause permanent irritation. Continual exposure may cause laryngitis, bronchitis, pulmonary edema and even death.
Diagram
Method
Four aspirin tablets (1.36 grams) were dropped into a clean conical flask. A pipette was placed in the 1.0 mol dm³ sodium hydroxide solution, and a safety filler was fixed onto the end. By winding it upwards, the pressure inside the filler came so low, the solution became sucked up inside the pipette. When the solution was sucked up past the 25cm³ mark, the safety filler was removed, and replaced by a thumb. By carefully lifting the thumb on and off the top of the pipette, the level of the solution dropped down. When this dropped so that the base of the discus was equal the 25cm³ mark, the solution was taken away and released into the conical flask on top of the aspirin tablets.
The Bunsen burner was then set up. It was placed onto a Bunsen mat, and the tubing was secured to a gas tap. A tripod was placed over this, and on this a gauze mat was placed. The gas was then turned on, and the Bunsen was lit using a burning splint. The valve was then opened, giving the hotter blue flame, which is more suitable for heating. The conical flask was placed on top, and was occasionally picked up with tongs and shaken around, to make sure it never quite boiled. After about 10 minutes, when the acetylsalicylic acid was fully hydrolysed, the conical flask was left to allow the solution to cool. This was important because if the hot solution was poured into a cold flask, it could cause the glass to expand and break.
After it cooled, the solution was poured into a standard flask. This was poured carefully, as all the moles from the conical flask needed to be transferred to the standard flask. It was okay to use distilled water to wash into the very bottom of the conical flask to get all the moles out, as the next step anyway was to dilute it to the 250cm³ mark, so the base of the discuss was on the line.
After the dilution, 25cm³ of the now 0.1mol dm³ solution was taken up in a pipette (once again using a safety filler) into a conical flask. A couple of drops of methyl orange indicator were then added, turning it a slightly yellow colour. The flask was placed on a white tile beneath the burette on the stand. The tap of the burette was closed, and a funnel placed on top. Holding the funnel carefully, 0.1 mol dm³ Hydrochloric acid was poured into the burette. When the burette was filled, the funnel was removed carefully, shaking of the last drop into the burette. The level of the discus in the burette was then marked down as a starting point in a table as shown below. The tap was then opened, and the hydrochloric acid mixed with the sodium hydroxide, slowly neutralizing it. The solution had to be picked up and swilled around for more accuracy. The nearer the solution got to being completely neutralised, the paler the indicator colour became. As it neared the end, the acid would be added drop by drop, until the indicator turned the slightest little bit pink. The discus level was then marked down as the end point of the experiment. The amount used to neutralise the solution could then be used to find out how many moles reacted with the aspirin. This experiment was repeated 4 times to get the most accurate results possible, and they were all recorded in the table below.
Results
Average = 17.5 + 17.7 + 17.6 + 17.7 = 17.625
4
17.625cm³ of Hydrochloric Acid is needed to neutralise the Sodium Hydroxide after it has reacted with the aspirin tablets.
Reliability of results
Calculations
Without Aspirin
Sodium + Hydrochloric Sodium + Water
Hydroxide Acid Chloride
NaOH + HCl NaCl + H2O
25cm³
1.0mol dm³
(1/1000) x 25 = 0.025
There are 0.025 moles in 25cm³ of 1.0 mol dm³ NaOH. Therefore, 0.025 moles of HCl are needed to react with it.
With Aspirin
(1/1000) x 17.625 = 0.017625
The titration showed that 0.017625 moles of HCl reacted with the NaOH. This means that only 0.017625 moles are left after the NaOH has reacted with the aspirin tablets.
0.025000 – 0.017625 = 0.007375
0.007375 moles of NaOH reacted with the aspirin
Acetylsalicylic + Hydroxide Acetate + Salicylic + Water
Acid ions ions ions
CH3COOCH4COOH + 20H CH3COO + HOC6H4COO + H20
There is a 1:2 ratio of acetylsalicylic acid to hydroxide ions
(0.007375/2) = 0.0036875
Therefore, 0.0036875 moles of acetylsalicylic acid reacted with the hydroxide ions. The relative molecular mass of the acetylsalicylic acid is 180
0.0036875 x 180 = 0.66375
0.66375 grams reacted, so the mass of each tablet that reacts is
(0.66375/4) = 0.1659375 grams
Each Tablet has a mass of 0.34 grams, so the percentage of acetylsalicylic acid in each tablet can now be found out
(0.1659375/0.34) x 100 = 48.805147
Therefore, in one aspirin tablet with a mass of 0.34 grams, there is approximately 48.8% of acetylsalicylic acid.
