IMAT 2015 question 50

Potassium nitrate was found to have a solubility of 120g in 100g H2O at 80°C, and 50g in 100g H2O at 25°C.

50g of H2O 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 seperated out to leave a saturated solution.

Using the information provided in this question, what is the minimum mass of H20 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?

Hi guys!
The answer is 70g, could someone explain how they solved this one?

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Hi!
In the original solution there will have been 60g of KNO3 dissolved in the solution.
To fully dissolve this amount at a lower temperature, there must be 60/50 x 100=120g of water present.
This means 70g must be added to the solution in order for all the potassium nitrate to be dissolved.
Ari has also made a video for this question under IMAT past papers questions.
Hope it helps:)

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