[andyman20]

Hi people

I understand all the other concepts on findin concentration but exceptfor this.
HOw do you find concentration of an ion of a molecule??

for example,,  in CaCl2
how do you find contraion of Ca 2+ ion and Cl - ion in a solution
if the solution was 0.2molL^-1

[Qazzian]

Well, that's easy.

The concentration you have is for the entire molecule, CaCl2.

There's 1 mole of Ca per mole of CaCl2 (you can see that in the molecular formula), and there's 2 moles of Cl per mole of CaCl2 (again, the formula).

So that means the concentration of Ca is the same as the concentration of CaCl2, and the concentration of Cl is twice the conenctration of CaCl2.

Just look at the mole ratios compared to the entire molecule.

[lemonoman]

Qazzian's got it right.  Unit analysis may be helpful here.

Let's say you have 4 mol CaCl2 / L.

4 mol CaCl2          2 mol Cl-           8 mol Cl-
-------------     ×   ------------     =  ------------
       L                   mol CaCl2              L

Notice how similar units cancel out