Find the error in this notebook entry and explain what the error is and how it c

Question

Find the error in this notebook entry and explain what the error is and how it can be corrected.  What information is missing from the notebook that might have prevented the error?

Titration of 3 different acids using NaOH

Acids titrated:

benzoic acid, formula: C6H5COOH, molar mass: 122.12 g/mol

malonic acid, formula: HOOCCH2COOH, molar mass: 104.06 g/mol

acetic acid, formula: CH3COOH, molar mass: 60.05 g/mol

indicator used: thymol blue, color (acid=yellow, base=blue) endpoint color: green (changing from acid to base)

Problem: find the concentration of the NaOH solution by titrating these 3 acids

Data:

mass of acid used

(g)

moles of acid used

(moles)

moles of NaOH

(moles)

volume of NaOH

(at endpoint) (mL)

concentration of NaOH

(mol/L)

acid

mass of acid used

(g)

moles of acid used

(moles)

moles of NaOH

(moles)

volume of NaOH

(at endpoint) (mL)

concentration of NaOH

(mol/L)

C6H5COOH 0.357 .00292 .00292 28.35 0.103 C6H5COOH 0.382 .00313 .00313 30.40 0.103 HOOCCH2COOH 0.349 .00335 .00335 65.12 0.0514 HOOCCH2COOH 0.328 .00315 .00315 61.23 0.0514 CH3COOH 0.188 .00313 .00313 30.37 0.103 CH3COOH 0.197 .00328 .00328 31.87 0.103

Explanation / Answer

The error is the concentration of NaOH obtained by titrating with Malonic acid. Please note that Malonic acid is a diprotic acid, i.e, it has two replacable hydrogens unlike the other two given acids. So it will have two end points. It can be corrected by properly noting down all the end points and making calculations likewise. The information that is missing is the data regarding the two end points. In the above table, only one end point data is given for Malonic acid and we do not know which one it is.

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