Question

How many grams of CaCl2 are needed to make 100ml of a 15% solution in water if only the dihydrated (CaCl2 •2H20) salt is available?

Answer 1

You need to re-read the problem and see how impossible it is; however, I think I know what you are asking. You want a 15% solution of CaCl2 but you must use CaCl2.2H2O to make the solution. If that's what you're asking, do it this way. And I'm assuming that is 15 mass percent.
For 15% CaCl2 you want 15 g CaCl2 in 100 g solution.
15 g CaCl2 x (molar mass CaCl2.2H2O/molar mass CaCl2) = grams CaCl2.2H2O needed.
Dissolve that many g CaCl2.2H2O in a little water and add H2O to make a total of 100 g solution.

Answer 2

I'm just as perplexed. Thank you for the help