# Finding mass of acetyl salicylic acid in an aspirin tablet

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#1
There must be something wrong with my calculation below.

Question:

25 cm3 of 1M NaOH added to an aspirin tablet. 25cm3 of distilled water also added. Mixture heated for 10 mins. Hydrolysed solution and washings transferred to a 250 cm3 conical flask and made up to the mark (250cm3).

25 cm3 of the hydrolysed solution is titrated against 0.1M HCl. The average titration is 12.13 cm3 HCl required to reach end point.

1 mol acetyl salicylic acid reacts with 2 mols NaOH
1 mol NaOH reacts with 1 mol HCL
Mr(ASA) = 180.15 g/mol

Attempt:

a) number of moles of NaOH originally added to the aspirin tablet is:
25*1/1000=0.025 mols

b) number of moles of NaOH remaining in 250 cm3 hydrolysed solution(not used to hydrolyse ASA):
=number of moles of HCl used to titrate against 25cm3 hydrolysed solution *10
=(12.13*0.1/1000)*10 =0.01213 mols

c) number of moles of NaOH used to hydrolyse the ASA
= a) - b)
=0.025-0.01213
=0.01287 mols

d) number of moles of acetyl salicylic acid hydrolysed by NaOH:
=number of moles of NaOH used to hydrolyse ASA/2
= 0.01287/2 = 0.00644mols

e) mass of ASA in the tablet:
=0.00644*180.15
=1.6g

however, the mass of ASA should be 300mg (as on the packet)........
There must be something wrong with my calculation.
0
6 years ago
#2
Hi.
I carried out back titrations to calculate the mass of pure acetylsalicylic acid in aspirin tablets for my AH investigation and I had a look through your calculations and done one myself. I calculated the mass of pure aspirin from the information above to be 1.159g (I'm not sure if you meant to write 1.16g?). Does the question definitely say one tablet or does it say a certain of number? I've seen a few questions like that... If not then it does seem correct. Also, what do you mean by the packet?
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