How much water must be added to 12 grams of a 90% iodine solution to produce a 25% iodine solution?
You would have to add 31.2 g of water.
Iodine dissolves in water only to 0.03 % concentration, so this is a purely hypothetical question.
90 % iodine means
So in 12 g of 90 % solution:
We have to add 31.2 g of water.