[firedamage]

Why CuSO4 need to disslove in H2O before add KI-Na2S2O3solution?
And the percipitate formed is 2CuI+I2?

[Arkcon]

why CuSO4+5H2O?

1. - That is not a descriptive enough title for your question.

2. - That is not how we describe the hydrated salt of copper sulfate, but rather how we start a chemical equation.  You know that, I've seen you write them out before.

Why CuSO4 need to disslove in H2O before add KI-Na2S2O3solution?

Presumably, you've done this experiment recently?  Don't assume that we all have.

And the percipitate formed is 2CuI+I2?

Possibly.  Why not write out a balanced chemical equation?  Reactants on once side, products on the other.  There are arrow icons on this forum for you to use.

[firedamage]

Here's the procedure of the experiment:
1)Weigh a 2.5 g sample of CuSO4 into a 250ml beaker and disslove it in 70ml of deionised water
2)add about 2.5M KI-2.5M Na2S2O3 solution into CuSO4 solution.When the brownish percipitate turns to pale violet,stop the addition.
3)weigh a piece of filter paper.place the filter paper in a filter funnel.filter the solution  into a 250ml conical flask.
3)remove the filter paper and place it in a beaker.leave the filter paper and preicpitate to dry and weigh the filter paper and precipitate.

#What is KI-Na2S2O3 solution?
#the brownish precipitate is CuSO4+H2O?
#the pale violet precipitate is 2Cu+(aq)+4I-(aq)=2CuI(s)+I2(s)?
#the precipitate in the filter paper i think is 2CuI+I2?

[firedamage]

can anyone help me pls...
 

[Astrokel]

#What is KI-Na2S2O3 solution?

Read the question again, there is no KI-Na₂S₂O₃ solutions, they are two different solutions. What you should be thinking is what is the significance of adding KI before reacting sodium thiosulphate with CuSO₄.

#the brownish precipitate is CuSO4+H2O?

Are you able to write the chemical equation for what is happening and see which precipitate is formed?

#the pale violet precipitate is 2Cu+(aq)+4I-(aq)=2CuI(s)+I2(s)?

Which?

#the precipitate in the filter paper i think is 2CuI+I2?

Be specific, don't quote 2 reagents or whole equation

Well it seems you don't understand the experiment you have done but take it slowly and think each steps and what could have been formed. Try to post what you think it is and we could confirm with you.

[firedamage]

the percipitate formed is 2CuI+I2

[Astrokel]

That is correct but i said quote one reagent only.