Would the following technique errors result in an incorrectly high incorrectly l
ID: 510119 • Letter: W
Question
Would the following technique errors result in an incorrectly high incorrectly low calculated mass percent of acetic acid in vinegar? Briefly explain. (1) A student properly followed Part I of the Procedure there was air in the pipet stem when he titrated the vinegar in Part II, causing some of the drops of NaOH solution to be much smaller than the standard size, (2) After standardizing the NaoH solution, a student rinsed the well plate but did not drain the excess water from the wells Thus, the wells to which he added the vinegar were wet. (3) A student failed to standardize his NaOH solution with KHP Instead, he calculated the NaoH solution concentration from the mass of NaOH his laboratory instructor dissolved to prepare the solution. Would the procedure you used in this experiment be suitable for the analysis of acetic acid cider vinegar? Briefly explain. A student measured 20 drops of vinegar and one drop of phenolphthalein solution into a well of her well plate. After she had added 35 drops of NaOH solution to the same well, the well was full, but she had still not reached the titration end point. She then took a clean pipet and transferred some of the titration mixture into a second well. She rinsed the pipet with a few drops of distilled water and transferred the rinse water into the titration mixture in the second well. Then she continued adding NaOH solution to both wells until she reached the end point in both wells. She then recorded the total number of drops of NaOH solution added to both wells and used it to calculate the data for her vinegar titration. Briefly comment on her procedure and the accuracy of her results.Explanation / Answer
Q.1.
(1.) Bubble in the pipette stem:
The bubble in the pipette stem would lead to lesser burette reading.
Say, for Y ml, the burette reading is supposed to be X cubic cm. But due to the presence of the air bubble in the pipette, the actual volume is lesser, i.e., it is Y-N (bubble occupies space, thus if pipette is supposed to have 10 ml liquid, it will have less than 10 ml.) but the reading is X-M cubic cm.
Thus, the bubble causes a discrepant burette reading which is lesser than it is supposed to be, in actual.
NOTE: Depending upon the concentrations of the acid and base used, the extent of discrepancy can be defined.
(2.) The residual, unwanted water in the plates would evidently DILUTE the NaOH.
And, the calculated molar mass would be very low if the burette had been rinsed with the deionized water and then filled with NaOH solution without rinsing with the NaOH solution first because the NaOH solution lines the burette so it suitably reacts with the acid solution. If it is rinsed with water without being rinsed with NaOH then the small particles of the base would be washed away which would create a fractional difference in the concentration of the titration.
Thus, the value would be affected and would turn out to be lesser than the expected value for the volume of NaOH in consideration.
(3.) NaOH is a hygroscopic compound which means it easily absorbs water from the air.
Thus, the mass of any solid NaOH sample that is measured will actually be NaOH plus water. The fraction of NaOH weighed is unknown, hence, the concentration of the solution from the supposed weight would give an incorrect value whereas, titration would provide a precise value.
Q.2. The procedures 'you' used in the experiment, it is not clear exactly which procedures is the examiner asking about. Please mention it clearly.
Q.3.
When distilled water is added on this erlenmeyer will change its volume, but won't change the number of moles of acid inside it, neither the initial volume you added of the acid solution. Sometimes it's even recommended that you add some water to make the color change more visible.
The aim of the titration is to find the volume of the base (of known concentration) required to neutralize the acid, and therefore, since you know the volume of acid added in the Erlenmeyer flask (the pipette volume), calculate the concentration of the acid solution.
Whereas water in the burette causes variations in the concentration of the base used, the reason why it is rinsed with the base is so there is a good precision titration. The Erlenmeyer flask can be rinsed only with distilled water since the volume of acid solution used for the calculation is constant.
Accuracy would not be compromised as much as expected.
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