Question

The solubility of NaNO3 is 143 g per 100 g of water at 75 °C and 92.1 g per 100 g of water at 25 °C. If 243 g of a saturated solution of NaNO3 at 75 °C is cooled to 25 °C, the mass of NaNO3 that will crystallize out is closest to:

Answer 1

This is a tricky and interesting question.

Data given:

Per 100 g of water, solubility of NaNO3 at 75^​o C = 143 g

at 25^​o C = 92.1 g

Since the solution used is saturated at 75^​o C it only has 143 g and obviously when it's cooled to 25^​o C the excess salt will precipitate. Thus we only need to consider 143 g of dissolved NaNO3 at 75^o​C and subtract the amount (92.1 g) of NaNO3 dissolved at 25^​o C to get the mass of NaNO3 that will crystallize out.

therefore

143 g - 92.1 g = 50.9 g

The mass of NaNO3 that will crystallize out is close to 50.9 g.