IMAT 2015 Q50 [Solubility and Dissolving]

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