Question

The solubility of a compound in water is 6.8g/100ml at 25°c calculate the amount of water required to crystallize 10grams of the compound if the pure compound is collected at 25°c what is the maximum possible yield percent?

Answer 1

I'm having a little trouble knowing exactly what you want; however, I think your compound has a solubility of 6.8 g/100 and you want to collect 10.0 g.
The solubility is 5.8 and you want to collect 10.0 so you must have enough solution that contains 6.8 + 10.0 = 16.8 g of the compound. How much solution must you start with? That's 100 x 10.0/6.8 = 247 mL. Now if you evaporate 147 mL of the 247 mL to leave you with 100 mL of solution, you will get 16.8-6.8 or 10.0 g of the compound.
% recovery is (10.0/16.8)*100 =? or about 60%.
Notice that you have not made it clear as to the final volume to be used and I've assumed 100 mL; however you could use a smaller final volume. For example, if you chose 50 mL final volume the numbers are
6.8 g/100 mL = 3.4 g/50 mL.
You will then need an inital volume that will accommodate 3.4g + 10.0 = 13.4.
Then 100 x (13.4/6.8) = 197 mL of solution.
Evaporating to 50 gives you 13.4-3.4 = 10.0 g
% = (10/13.4)*100 = about 75%.

Finally, if you want to reduce the final volume to zero mL to get all 10.0 how much initial solution to you need? That's 100 x (10.0/6.8) = 147. Now you have 10.0 g of the compound in solution, evaporate all of the water and you will collect alol 10.0 g which will be 100%, at least theoretically.
I hope one of these solutions is what you are looking for.If not, clarify and we can go at it again.