Titrations, need urgent help

i have questions to answer for a practical on titrations:

why do we need to make up a solution of a similar concentration as the unknown one?

why do we need to rinse the beaker 2 or 3 times with small amounts rather than once with a bigger amount

why do we record the volume to the nearest 0.05cm^3

why do we continuously swirl the flask?

Reply 1

Want_To_Achieve

Original post by okelydokely

I can answer the last question (kinda). We swirl the flask so that we can record a colour change quicker than if we didn't swirl it. This is because the solution with the known concentration that is in the burette is reacting with the solution in the flask so swirling the flask makes the process quicker so that a colour change is recorded more accurately.

This is what I think bear that in mind.

Reply 2

okelydokely

Original post by Want_To_Achieve

thank you

Reply 3

Want_To_Achieve

Original post by okelydokely

thank you

No problem

Reply 4

ChemistryWebsite

Original post by okelydokely

1) If the titrant is much weaker than the sample, you will need a much larger volume. This may require you to refill the burette, perhaps several times, for a single titration - leading to errors. Additionally the end point may not be very distinct with a very dilute titrant.
If the titrant is much stronger than the sample, the volume titrated will be quite low, causing error. Additionally, that last drop that triggers the end point will contain a relatively high number of moles (relative to the sample) so the end point will not be precise.
Using titrant with similar conc to the sample is a good balance of precision and distinctiveness of endpoint whilst keeping errors to a minimum.

2) Each rinse will leave a little bit of the rinse water in the beaker, and this will contain a small amount of your reagent. If you repeat the rinse with fresh water, this will flush more of the reagent out, leaving even less in the beaker. The more repeats you do of this, the less reagent remains in the beaker.

Try it with a couple of milk bottles with a little bit of milk remaining in each. Rinse one bottle once with some water. Rinse the other with several smaller amounts of water, totalling the same amount as used for bottle 1. Look at the residue in each bottle, bottle 2 will be clearer.

3) Most burettes are marked in intervals of 0.1cm3, and it is considered that you can estimate half that volume between each graduation. Making this estimate increases the precision of the titre value. But trying to estimate "closer", e.g. to the nearest 0.02cm3 would just be kidding ourselves about how good our eyes and visual perception are.

4) When nearing the end point, but not swirling, the colour of our endpoint will remain for a while. This may make us think we have reached the endpoint before we actually have, or may lead to us overshooting the endpoint.
Swirling whilst adding titrant is more likely to cause us to slow / stop the titration at the right points and we will get a more accurate titre value.

Titrations, need urgent help

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