chemistry - titrating

In the titration, burette contains solution X and the conical flask contains a fixed volume of solution Y which is transferred by using a pipette. In calculations:

No. of moles of X = (Molarity of X) x (Volume of X)
No. of moles of Y = No. of moles of Y transferred using pipette

Consider the burette:

No. of moles of X = (Molarity of X) x (Volume of X)
Note that the molarity of solution X is taken in the calculations. All steps in the titration should NOT affect the concentration of solution X.

(1) If the burette is rinsed with water, some water left in the burette. Therefore, when the burette is filled with solution X, solution X is diluted and the its molarity becomes smaller. This leads to an error.

(2) If the burette is rinsed with solution X several times, the concentration of the solution left in the burette would be very close to that of solution X. Therefore, when the burette is filled with solution X, no dilution occurs.

Conclusion: The burette should be rinsed with solution X before titration.

Consider the conical flask:

No. of moles of Y = No. of moles of Y transferred using pipette
Note that the number of moles of Y transferred by using the pipette is taken in calculations. All steps in the titration should not affect this value.

(1) If the conical flask is rinsed with solution Y, a small amount of solution Y is left in the conical flask. Therefore, the number of moles of solution Y in the conical flask becomes greater than the number of moles of solution Y transferred by using the pipette. This leads to the error.

(2) If the conical flask is rinsed with water, a small amount of water is left in the conical flask. The water left does not affect the number of moles of solution Y that transferred by using the pipette. Although solution Y is diluted, the total volume is increased at the same time. The total number of moles of Y is unchanged (no. of moles of Y = molarity x volume).

Conclusion: The conical flask should be rinsed with water before titration.

2008-07-01 19:57:58 補充：
Titration 通常是中四的課程。若使用 burette 和 pipette 而不知道 molarity (molar concentration)，是有點奇怪的。

Burette washed with solution X.

->
The burette will be added solution X after cleaned.
If the burette is cleaned with water, the remained water dilutes the solution X
which is added to the burette.

Answer: Using solution X to clean the burette will not change the molar
concentration of the solution X which is added to the burette.
Opposite, cleaning with water changes the molar concentration of solution X
which is added to the burette.


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Conical flask washed with distilled water.
->
The conical flask will be added solution Y after cleaned.
If the conical flask is cleaned with solution Y, the remained solution Y raises
the molar mass of the solute. Hence, it will affect the test.

Answer: Although it raises the volume and the molar concentration of the
liquid in the conical flash, it does not affect the test.
Opposite, using solution Y to clean the conical flask affects the test.