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# Aspirin back titration calculation

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1. I am having trouble with doing a calculation for a back titration.

I reacted 1.50g of impure aspirin (where I am assuming the impurities are salicylic acid) with 1g sodium hydroxide in 50cm3 water. I then titrated this solution with 0.5M hydrochloric acid, with a titre of 22.70cm3.

Do I need to consider salicylic acid reacting with sodium hydroxide, and if so how do I include this in my calculation?
2. The active agent in Aspirin is the salicylic acid, it is not an impurity.
1 mole NaOH has mass 40g. 1 g will have 0.025 moles in it.
the total volume when the HCl has gone in is 72.7 cm3
the number of moles HCl used in 22.7 cm3 is n=v/1000 x concn number =22.7/1000 x 0.5 = 0.0114
ASA has one carboxylic acid group.
there must be a TOTAL of 0.025 moles acid to perfectly neutralise 1 g of NaOH. 0.0114 gets supplied by the HCl, so the rest of it is the ASA. This means there are 0.0137 moles of ASA.
ASA has a formula of C7H6O3. So this is a MM of (12 x 7) + (1 x 6) + (16 x 3) = 138
so 1 mole has a mass of 138 g, so 0.0137 moles has a mass of 1.89 g which is MORE than the mass of impure aspirin.
all I can assume is that the aspirin was not broken down fully and so only part of it actually neutralised the NaOH
3. (Original post by benion)
The active agent in Aspirin is the salicylic acid, it is not an impurity.
1 mole NaOH has mass 40g. 1 g will have 0.025 moles in it.
the total volume when the HCl has gone in is 72.7 cm3
the number of moles HCl used in 22.7 cm3 is n=v/1000 x concn number =22.7/1000 x 0.5 = 0.0114
ASA has one carboxylic acid group.
there must be a TOTAL of 0.025 moles acid to perfectly neutralise 1 g of NaOH. 0.0114 gets supplied by the HCl, so the rest of it is the ASA. This means there are 0.0137 moles of ASA.
ASA has a formula of C7H6O3. So this is a MM of (12 x 7) + (1 x 6) + (16 x 3) = 138
so 1 mole has a mass of 138 g, so 0.0137 moles has a mass of 1.89 g which is MORE than the mass of impure aspirin.
all I can assume is that the aspirin was not broken down fully and so only part of it actually neutralised the NaOH
But reacting ASA with sodium hydroxide will hydrolyse the ester? Also salicylic acid will also react with sodium hydroxide. Surely this would mean 1 mole of ASA reacts with 2 moles of NaOH, and 1 mole of salicylic acid reacts with 1 mole of NaOH? Please tell me if I'm wrong. Thanks!
4. (Original post by HT12345)
But reacting ASA with sodium hydroxide will hydrolyse the ester? Also salicylic acid will also react with sodium hydroxide. Surely this would mean 1 mole of ASA reacts with 2 moles of NaOH, and 1 mole of salicylic acid reacts with 1 mole of NaOH? Please tell me if I'm wrong. Thanks!
Sorry I have been thick and have just calculated the whole thing on salicylic acid which is C7H6O3. The salicylic acid is the active drug in Aspirin.

I have looked up the hydrolysis of ASA and it breaks down to SA and ethanoic (acetic) acid. So you are correct, there are now TWO -COOH groups, one on the SA and one on the ethanoic.

The hydrolysis of the ester will only happen if you reflux it and you do not state this in the Q. If it was not done under reflux, the hydrolysis will be too slow to be significant.

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Updated: April 11, 2016
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