consider each of the following and decide what effect it would have on youe expe

Question

consider each of the following and decide what effect it would have on youe experiental molarity . Will the error cause your answer to be higher or lower than it should be >or will it have noo effect at all. Explain

1)adding the vinegar to an Erlenmeyer flask that already has 2 mL of deionized water in it?

2)using higher concentration of NaOH in your calculation instead of the correct concentration?

3)forgetting to rinse down the inside wall of flask with deionised water during and at the end of titration?

4)reading the Naoh initial buret reading as 1 mL lower than correct value?

5)why should you not plan to start the titration with the acid and base burette filled exactly th the zero mark?

6)why should you titrate into an Erlenmeyer flask rather than into a beaker?

Explanation / Answer

Ans. 1. At the end point of titration, the number of moles of acetic acid is equal to number of moles of NaOH.

Presence of 2.0 mL water in the flask prior to addition of vinegar do dilute the solution. However, the total number of moles of acetic acid before and after adding distilled water remains the same.

So, dilution does not affect the number of moles of acetic acid. So, endpoint remains the same irrespective of amount of water present in the flask.

Therefore, it does not affect the molarity of acetic acid.

Ans. 2. Using a higher concentration (incorrectly) of NaOH in calculation than actual value gives higher concertation of acetic acid.

Consider, M1V1 = M2V2      - at the end point

Where, “1” denotes acetic acid and “2” denotes NaOH. V= velocity, M = molarity; and

Number of moles = Molarity x Volume (in L). However, when both side of equation has volume in terms of mL, there is no need to convert volume in L.

            Or, M1 = M2V2 / V                           --- M1= molarity of acetic acid.

Note that, molarity of acetic acid is directly proportional to that of NaOH. So, increasing NaOH concertation (incorrectly) while keeping other variables constant gives higher value of acetic acid molarity in vinegar.

Ans. 3. The inside wall of the flask is rinsed to bring any drop of solution (specially acetic acid being titrated) splashed on walls back into the solution. So, if any drop of acetic acid remains sticking onto wall, less NaOH shall be required to reach the endpoint. Thus, less NaOH value gives lower concertation of acetic acid than the actual one.   

Ans. 4. M1 = M2V2 / V                                 --- from Ans 2.

Molarity of acetic acid is directly proportional to the volume of NaOH consumed to reach the end point. So, a reading lower NaOH volume than the actual used up gives lower concentration of acetic acid.

Ans. 5. It’s crucial to let few mL of titrant filled in burette flow through as a discarded volume. It serves two purposes- I. it removes any air bubble trapped in the nozzle of the burette, and II. It fills the dead space between the tip of nozzle and screw (opening-closing screw) so that exact volume being dispensed from the burette can be recorded precisely.

So, there is always need to discard the titrant before starting the titration. And it is not wise to re-fill the burette exactly 0.0 reading as it consumes extra time, compromises accuracy without any better output of readings.

So, it’s better not to plan starting titration with 0.0 reading, it saves the time while affecting the result.

Ans. 6. Addition of NaOH to the flask requires simultaneous swirling the reaction mixture, too. Beaker as a wider mouth and raise potential risks of spillage/ splashes outside- thus causing errors to creep. Moreover, gripping a beaker during titration is default due to its greater width. Erlenmeyer flask has a narrow mouth (neck) and wide bottom. So, it prevents splashes of the reaction mixture from escaping the vessel. Also, because of its narrow neck, it can be handled much conformably than a beaker.

Therefore, to prevent splashes and for comfortable handling, Erlenmeyer flask is preferred over beaker during titration.

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