Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: MW2121 on March 02, 2019, 04:10:19 AM
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For school, we performed a back titration involving one brand of aspirin and were told to determine the quantity of acetylsalicylic acid in a single tablet. sodium hydroxide was added in excess then the solution was heated. the excess unreacted NaOH was then reacted with HCl. my calculations are as follows:
V(NaOH) = 0.04085L
c(NaOH) = 0.111M hence n(NaOH) total = 0.00435 moles
v(HCl) = 0.01778L
c(HCl) = 0.0582M hence n(HCl) = 0.001035 moles
number of moles excess = number of moles titrated of HCl
hence 0.001035 moles didn't react with acetylsalicylic acid
the number of moles of NaOH that reacted with acetylsalicylic acid is 0.00435-0.001035 = 0.003495
therefore 0.003495 moles of NaOH reacted with the acetylsalicylic acid
the mole ration between NaOH and acetylsalicylic acid is 1:2
meaning the number of moles of acetylsalicylic acid is 1/2 x 0.003495
which is = 0.0017475 moles
Molar mass acetylsalicylic acid = 180.158g/mol
0.0017475 x 180.158 = 0.314g or 314mg
this does not make sense considering I only added 300mg of crushed aspirin powder in the first place, meaning that I would have a percentage purity of 105%
if anyone can see any obvious errors on my part or could suggest any reason for this ridiculous result, I would be extremely appreciative :))
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I think your calculation is ok, I think the error is in the datas.THe reactingtime of NaOH with the aspirin was to short. It is not only a neutralzation also the ester group as to be split.
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But in that case, the amount of aspirin must be even greater...?
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Incomplete hydrolysis leads to an underestimation of acetylsalicylic acid content (more NaOH is titrated with hydrochloric acid).
But if NaOH contains some sodium carbonate (which happens with older NaOH solutions), the titration with HCl (against phenolphthalein as an indicator) shows only the half of the NaOH converted to sodium carbonate and the acetylsalicylic acid content may be overestimated.
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Incomplete hydrolysis leads to an underestimation of acetylsalicylic acid content (more NaOH is titrated with hydrochloric acid).
But if NaOH contains some sodium carbonate (which happens with older NaOH solutions), the titration with HCl (against phenolphthalein as an indicator) shows only the half of the NaOH converted to sodium carbonate and the acetylsalicylic acid content may be overestimated.
ok, that sorta makes sense, but how does the conversion of NaOH to NaHCO3 mean that the acetylsalicylic acid content is overestimated? wouldn't it have the same effect as incomplete hydrolysis of underestimation?
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This is a simple stoichiometry
Assume 1 mole of pure ASA (acetylsalicylic acid) and 3 moles of NaOH. Simulate 100 % or 50 % of hydrolysis and calculate results according to your formula.
Then convert 0.25 mole of CO2 with 0.5 mole of NaOH and do complete hydrolysis of 1 mole ASA. During titration o.25 mole of Na2CO3 is converted to NaHCO3 (complete conversion to NaCl needs pH less than 7). and calculate results.
Note - reaction CO2 with NaOH gives you Na2CO3 to the momet when all NaOH will be reacted. Since then more carbon dioxide forms sodium hydrogen carbonate.
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How much NaOH do we realy need. Acetylsalicylic acid is an Ester and an acid, after hydrolysis we have also a phenolic function.
1. CH3COO-C6H4COOH + NaOH => CH3COO-C6H4COO- + Na+ + H2O
2. CH3COO-C6H4COO- Na+ + NaOH => CH3COO- Na+ + HOC6H4COO- Na+
3. HOC6H4COO- Na+ + NaOH => Na+ +-O-C6H4COO- + Na+
What is right now. According this the ratio is 1:3 and not as mentioned above 1:2
The calculated result would be then 209,8 mg
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How much NaOH do we realy need. Acetylsalicylic acid is an Ester and an acid, after hydrolysis we have also a phenolic function.
1. CH3COO-C6H4COOH + NaOH => CH3COO-C6H4COO- + Na+ + H2O
2. CH3COO-C6H4COO- Na+ + NaOH => CH3COO- Na+ + HOC6H4COO- Na+
3. HOC6H4COO- Na+ + NaOH => Na+ +-O-C6H4COO- + Na+
What is right now. According this the ratio is 1:3 and not as mentioned above 1:2
The calculated result would be then 209,8 mg
Yes, with NaOH the phenolate can be formed.
No, during the titration, the phenolate is treated as NaOH. Hence the calculations of MW2121 are correct.
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If so, why is then the ratio 1:2 used?
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ASA during hydrolysis form salicylate and acetate (the same amount of moles). This is true for titration with phenolphthalein indicator.
Nonhydrolyzed ASA forms a salt with 1 mole of NaOH.
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Yes this is 1 and 2, and I mentioned the built up of phenolat what means 3.
But nevertheless this takes place:
http://www1.lasalle.edu/~prushan/BackTitration-lab4.pdf
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Check acidity of phenol. What happens to phenolate at pH of 8 (phenolphthalein becomes colorless)
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Yes this is 1 and 2, and I mentioned the built up of phenolat what means 3.
But nevertheless this takes place:
http://www1.lasalle.edu/~prushan/BackTitration-lab4.pdf
The procedure given in above link is horrible in this point
Shutdown procedures
IMPORTANT NOTE: Your NaOH solution must be saved upon completion of the experiment; you
will use it again in future experiments
The diluted solution of NaOH reacts with carbon dioxide from the air and before next using should be checked for sodium carbonate content (and eventual correction). Otherwise, the ASA content will be overestimated.
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In this experiment Aspirine and NaOH in excess was mixed and heated. I think the pH will be more as 8. Later the titration took place with the remaining NaOH and HCl.
The difficulty could be an hydrolysis of the phenolat again at pH 8. ??? Maybe then only Neutralisation and split of ester is mentioned.
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In this experiment Aspirine and NaOH in excess was mixed and heated. I think the pH will be more as 8. Later the titration took place with the remaining NaOH and HCl.
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The difficulty could be hydrolysis of the phenolat again at pH 8. Maybe then only Neutralisation and split of ester is mentioned
At pH ~8 only free phenol exists (there some exceptions, eg nitrophenols, some fluorophenols). Hence for simplicity, we can neglect the formation of a phenolate.
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I understood.
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Can anyone explain why we used phenolphthalein indicator in this titration when it has a pH range of 8.2=10??? I'm very confused and think it may be a source of error
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Just look at the numbers of problem sent by MW2121
After complete hydrolysis of ASA you can expect concentrations of sodium salicylate and sodium acetate between 0.01 to 0.02 M.
What may be the pH of these salts present in the solution individually? My guess - below 8 and about 8.5, respectively. You can check by calculations).
When these salts are taken together in solution and additionally Na+ ions in the solution will be in excess (from NaOH or later NaCl), then hydrolysis will take place to a lesser extent and we can expect pH of this solution slightly below 8. The color change of phenolphthalein is above 8.2, so in titration, these salts will not participate. And that's what we want.