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Methanoic acid ($\ce{HCOOH}$) is found in stinging nettles. In an analysis of the concentration of methanoic acid in a solution extracted from nettles, the solution was titrated with a standard solution of potassium hydroxide. Aliquots of the methanoic acid were transferred to a conical flask and titrated against the potassium hydroxide solution in a burette.

Which one of the following would cause the calculated concentration of methanoic acid to be higher than it actually is?

A Rinsing the conical flask with deionised water
B Rinsing the burette with deionised water
C Rinsing the pipette with deionised water
D Rinsing the conical flask with potassium hydroxide solution

For this question, im not sure if A and B are correct answers. The book only has B.

For A: if you rinse the conical flask, would this mean that there would be a greater volume in the flask after the titration than expected? Therefore when you have a lower concentration?

For B: if you rinse the burette with deionised water, this would dilute the $\ce{KOH}$, therefore causing a greater titre required. This means a greater mole of $\ce{KOH}$ and hence acid. Would this then lead to a greater concentration?

So B is the only correct answer, is my reasoning correct?

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Option A implies that the rinse residual would dilute the methanoic acid that would be added to the flask after rinsing and thus your titration would indicate that the acid is a LOWER concentration than the calculated value.

You are correct in your assessment of Option B.

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  • $\begingroup$ That is wrong! Excess water in the conical flask will have no influence whatsoever. Volumetric analysis is the quantitative determination of the amount of substance. The volume you read is from when you take the aliquot with the pipette. The water you add will not change that reading. $\endgroup$ – Martin - マーチン Sep 27 '17 at 6:45
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A Excess water in the conical flask will have no influence whatsoever* (in first order approximation). Since you are determining the the amount of substance of methanoic acid with the titration, the volume is irrelevant. The volume you use to determine the concentration is from when you take the aliquot with the pipette.
This option is therefore incorrect.

B You are correct, the excess water left in the burette will dilute you potassium hydroxide solution, causing you to use more of it, and therefore also reading a higher amount of substance of methanoic acid than would be correct. Obviously this leads to a higher concentration.
This is a correct option.

C Rinsing the pipette with water will cause you analysis solution to be diluted. Therefore the calculated concentration would be lower than the actual concentration.
This option is therefore incorrect.

D Rinsing the conical flask with potassium hydroxide solution will lead to having some of the methanioc acid already neutralised. You will use less of your analysis solution, therefore calculating a lower concentration.
This option is also incorrect.


In order to achieve the highest accuracy, you should

  1. rinse the burette with the potassium hydroxide solution you use to analyse.
  2. rinse the conical flask with plenty of deionised water (to remove any residues).
  3. rinse the pipette with the solution which is to be analysed.

* If it is too much water, then the volume significantly changes. This will cause the indicator substance to react ever slightly differently, which might lead to you not seeing the equivalence point early enough. This could lead to reading a higher concentration.
However, you eyes are a larger source of error if it is a few drops of water.

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