Potassium nitrate was found to have a solubility of 120 g in 100 g of water at 80 °C and 50 g in 100 g of water at 25 °C.
50 g of water was heated to 80 °C and solid potassium nitrate added until the solution was just saturated.
The solution was then cooled to 25 °C when solid potassium nitrate separated out to leave a saturated solution.
Using the information provided in this question, what is the minimum mass of water that must now be added to the mixture of the solution and the solid in order to make this solid potassium nitrate redissolve at 25 °C?
A. 20 g
B. 140 g
C. 120 g
D. 70 g
E. 190 g
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If the solubility of Potassium Nitrate is 120g in 100g of water at 80 degrees Celsius then we can compute how many grams have to be in 50 g of water instead: P= Potassium Nitrate, W= Water
P: W
120g is to 100g
x is to 50g
X must be 60g of Potassium Nitrate.
Then, when it was cooled to 25 degrees Celsius. We use the solubility at 25 degrees to find the amount of water, since we already know the mass of Potassium Nitrate present, we need to know how much water we need to dissolve it fully at this temperature.
P: W
50g to 100g
60g to x
x is 120g of water.
Therefore we need 120g of water in total to dissolve 60g of Potassium Nitrate at 25 degrees Celsius.
Remember though, we already have 50g of water, which means we only need to add 70g more. That makes the answer D.
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