Question

Complete the following assignment on this sheet or separate paper (not spiral wound paper). Your solutions should show all work presented neatly and in pencil. Make sure you clearly indicate your answers. Using the figure, an aqueous KCI solution containing 43.3 g of KNO_3 per 120.3 grams of water is cooled from 49 degree C to 23 degree C Explain, using appropriate terminology and as specifically as possible, what happens and will be observed during this cooling process.

Answer 1

Cooling i.e., decreasing the temperature decreases the solubility of the salt.

As temperature decreases, the average kinetic energy of the water molecules decreases as well. This implies that the water molecules will be less effective at overcoming the ionic bonds that are holding the solute particles together, and thus less effective at breaking apart the solid's crystal structure.

As a result, fewer ions, will be hydrolyzed, i.e. split from the crystal structure and solvated by the water molecules. and remaining will be precipitated as solid.

At 49°C:

The solubility curve shows that at 49°C, KNO₃ has a solubility of about 86 g/100 g H₂O and KCl has a solubility of about 41 g/100 g H₂O. This shows that at 49°C, the solubility of KNO₃ is more than KCl. So, KNO₃ dissolves first and KCl dissolves next.

At 49°C, 86 g KNO₃ requires 100 g H₂O

43.3 g KNO₃ requires = (43.3 / 86) x 100 = 50.3 g H₂O

Remaining water = 120.3 g - 50.3 g = 70 g

At 49°C, 41 g KCl dissoves in 100 g H₂O

Amount of KCl dissolves in 70 g H₂O = (41 x 70) / 100 = 28.7 g

At 49°C, the solution contains 43.3 g KNO₃ & 28.7 g KCl in 120.3 g H₂O

At 23°C:

The solubility curve shows that at 23°C, KNO₃ has a solubility of about 33 g/100 g H₂O and KCl has a solubility of about 33 g/100 g H₂O. This shows that at 23°C, the solubility of KNO₃ is equal to KCl. So, both KNO₃ and KCl dissolves next.

At 23°C, 33 g KNO₃ dissoves in 100 g H₂O

Amount of KNO₃ dissolves in 60.15 g H₂O = (33 x 60.15) / 100 = 19.8 g

At 23°C, 33 g KCl dissoves in 100 g H₂O

Amount of KCl dissolves in 60.15 g H₂O = (33 x 60.15) / 100 = 19.8 g

Precipitate of KNO₃ = 43.3 - 19.8 = 23.5 g

Precipitate of KCl = 28.7 - 19.8 = 8.9 g

At 23°C, the solution contains 19.8 g KNO₃ & 19.8 g KCl in 120.3 g H₂O and 23.5 g KNO₃ & 8.9 g KCl as precipitate.