the solubility of KNO3 is 32g/100g of water at 20oc and 62g/100g of water at 40oc. What will you observe if a saturated solution of KNO3 at 40oc is cooled down to 20oc?

Dear Student,

Potassium nitrate(KNO3) has a solubility of 32/100 g of water at 20C and of 62/100 g at 40C.
At 20C, dissolve as much as 32 g of solid potassium nitrate in water without the solution becoming saturated.
Once 32 g of the solution becomes saturated, which means that the solution can't dissolve any more solid.
At 40C dissolving less than 62 g of potassium nitrate per 100 g of water will result in an unsaturated solution.
At the 62 g, the solution will become saturated.
So,100 g of saturated solution at 20C.
100 g of solution contains as much dissolved potassium nitrate as possible.
This solution contains
100g solution x 32g KNO3/(32+100)g solution = 24.24 g KNO3
Now, solubility of potassium nitrate is given per 100 g of water. 
To calculate how much water you have in this initial solution
mwater =100g - 24.24 g = 75.76 g
To determine how much potassium nitrate can be dissolved in 75.76 g of water at 20C in order to make a saturated solution, i.e. have the maximum amount of dissolved potassium nitrate possible.
75.76g water x 62 g KNO3n / 100g water = 46.97 g KNO3
The amount of water contained only hold 46.97 g of dissolved potassium nitrate.
The rest will crystallize out of solution
mcrystallize = 75.76 g - 46.97 g = 28.79 g

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The crystals will be formed of KNO 3 and H2O
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- 30 g of KNO 3 and H2O
as the 32g of KNO3 will completely dissolve in 100g of H2O



CHEERS!
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