Titration(washing of burette)

2012-10-17 7:22 am

Why diluted solution in burette should not be used for titration?

During titration,the washed conical flask need not be dried before use.As I know, the reason is that the addition of water won't affect the no. of mole in the conical flask, which uses up the same amount of titrant whether it is dried or not.That means the molarity is not important. But howcome,the burette and pipette need to be washed with water following by the solution it is to deliver? I know that the solution will be diluted if water is left and the molarity will change, but why does this change in molarity matters while that in the conical flask doesn't matter?

Another question,is it always true that the conical flask contain a solution with UNKNOWN molart while that in the burette has KNOWN molarity?

回答 (1)

胡雪8°

2012-10-17 10:58 pm

✔ 最佳答案

1.
No. of moles of the solute of the solution in burette
= (Molarity of the solution in burette) x (Burette reading in dm³)
If the solution is diluted in the burette, the burette reading is not changed,but the molarity of the solution in the burette becomes smaller as the solutionis diluted. Therefore, the actual numberof moles of the solute of the solution in the burette is smaller than the calculatedone.

2.
No. of moles of the solute of the solution in conical flask
= (Molarity of the solution in pipette) x (Pipette reading in dm³)
From the above expression, the number of moles of the solute of the solution inthe conical flask is determined before the solution is transferred into theconical flask. When the solution is diluted in the conical flask, the number ofmoles of the solute in the solution is NOT affected.

3.
The solution with unknown molarity can be placed neither in the burette or theconical flask, while the solution with known molarity can be placed neither inthe conical flask or the burette.

參考: 胡雪

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