titration question

Pipette:
Transports 25cm^3 of the standard solution
If it contains water before, it will transport less than 25cm^3
Which means the no. of solute is reduced
Thus the concentration will change

Burette:
If it contains water before using, the volume of solution will be not accurate.
For example, if there is 2cm^3 water inside and then you pour some solution into it, then you will think that there is x cm^3 of the solution. But actually there is (x-2) cm^3 of the solution. Then the volume used to titrate will be not accurate

Conical flask:
It is used to contain solution. As you transfer the standard solution to it, the no. of solute is the same although the conical flask contains some water.
For example your standard solution contains 1g of NaCO3
It will not be changed although the conical flask contains water, there will also be 1g of NaCO3 solute in the solution

2012-06-03 22:05:29 補充：
The important thing is that the no. of solute is accurate.

The solution in the conical flask is a standard solution, ie. its molarity is known; also it's always 25cm^3, therefore its volume is known too. with these pieces of info, we can also calculate its no. of moles.

The titration process is just to check how much of the solution in the burette is needed to react completely with the standard solution, so as to deduce the burette solution's molarity. To achieve this, we need to use acid-base indicator to check if its complete or not.

Even though the solution in the standard solution is diluted, the amount of burette solution needed to react with it is still the same, as the no of moles of standard solution is still the same.

Then by calculating the mole ratio, we know the no of moles of burette solution needed for complete reaction. Together with the volume acquired from titration, its molarity can hence be calculated.

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