[davidjack]

I want to prepare 100mM stock solution of Hg(ClO)2.xH2O..

  here i wonder in what way we have to take the M.wt of this salt..

 for anhydrous form thy mention 399.45. degree of hydration also thy mention in sigma 3-5

 but i want exact conc...

help me to und this...
 thanks in advance..

[vex]

With an unknown molecular weight, unfortunately you're always stuck with an uncertain final concentration. There are two options available to you.

1) If it's easy to do so, prepare the anhydrous salt and use that. I can't imagine that this is going to be easy, so this probably isn't the route to take.

2) You can approximate the molar mass of the salt with the unknown composition, and then try to prepare a solution of the concentration that you want. Then, once your solution is finished, take an aliquot and perform some kind of titration (maybe a redox titration for Hg²⁺) to determine the actual concentration of the salt in your solution. From there, you can either modify your next experiment as necessary, or dilute the solution to get the concentration that you want.