The question of Titration , please tell me ^^

The reason for not using the solution of unknown concentration to wash the conical flask is to avoid contamination. If you wash the conical flask with the solution it would affect the concentration of the substance in the flask, and therefore affecting your result. The only apparatus you should wash out with any solution is the burette in order to avoid dilution. Whereas, the conical flask should be wash by deionised(distill) water.

because that unknown concentration solution will contaminate the subsequent solution that you are going to add into the conical flask by mixing up two DIFFERENT concentartion of solution.
As a result,the result obtained will not be accurate la~~