Question

The solubility of potassium nitrate, KNO3, per 100.0 g of H2O is 31.6 g at 20 ºC, and 85.5 g at 50 ºC. How much of KNO3 can be recrystallized if 184.0 g of KNO3 is dissolved at 50 ºC in 225.0 g of H2O and the solution is then cooled to 20 ºC producing solid KNO3 and a saturated solution of KNO3 in water. Assume that necessary precautions were taken to avoid evaporation of any water.

a. 121.3 g

b. 71.1 g

c. 53.9 g

d. 112.9 g

e. 98.5 g

Answer 1

Option d. 112.9 g is the answer.

At 50 deg C, 100 g of water can dissolve 85.5 g of KNO3.

Hence, 225.0 g of water can dissolve of KNO3. But we have added only 184.0 g of KNO3. Hence, all of added KNO3 will remain in solution at 50 deg C. Now solution is cooled to 20 deg C.

At 20 deg C, 100 g of water can dissolve 31.6 g of KNO3.
Hence, 225.0 g of water can dissolve of KNO3.

Thus, out of added 184.0 g of KNO3, 71.1 g will remain dissolved and KNO3 will crystallise out.