November 29, 2021, 04:09:53 AM
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Topic: Acid-base titration in lab classes  (Read 134 times)

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Offline BlueRiddle

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Acid-base titration in lab classes
« on: November 06, 2021, 05:36:16 PM »
I have an issue with properly performing acid-base titration. Lacking a better description, I'm failing miserably.

In an exercise, we have to perform a titration of a solution of CH3COOH using an NaOH solution. Now, we also don't know the concentration of the NaOH solution, only that it is around 0.1 mol/dm3, so first we have to figure that out with a solution of HCl with a known exact concentration.

So, I rinse a burette with distilled water first, and then NaOH. I rinse an erlenmeyer flask using distilled water, a pipette using the HCl, and then I add 25ml of the HCl into the flask, as well as a few drops of methyl red, and then slowly titrate until the first change of colour. I write down the volume I used, repeat and then calculate the mass of pure NaOH in the solution based on the volume used before the change of colour.

Knowing the concentration of NaOH, I then rinse the flask with more distilled water and refill the burette before adding in CH3COOH and a few drops of phenolphthalein, and titrating using that, writing down the volume used and using that to calculate the mass of pure CH3COOH in the solution, which was what we actually needed to calculate.

I give my result to the professor. 6% error, need under 3% to pass, failed. I'm pretty sure the mass I got was less than the actual mass of the sample.

Had another class where we were supposed to do titration, using an aqueous solution of Na2CO3. 23% error, failed. Failed the whole lab, in fact, as we're only allowed to retake one exercise.

I don't know the result for my most recent class, but since it was titration again, this time complexometry using EDTA, I'm prety sure I'm going to fail that as well as I still have no idea what i could be doing wrong.

Offline shchavel

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Re: Acid-base titration in lab classes
« Reply #1 on: November 07, 2021, 02:46:21 AM »
Hello, BlueRiddle!
The best way to find out root cause of you problem is do titration with your professor (or someone who passed that labs).
Sometimes just have a observer help you to work more carefull.
Also try to wash a burette by water twice, and by your titrant once, before filling it with titrant. (I always do that before titrations, and it's really helps).

Ask to help you more suc—Āsessfull students! Don't be shy!)

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