If 60 grams of ammonium chloride is dissolved in 100g of water at 85oC to create a supersaturated, at what temperature will the solution need to be cooled to to create a saturated solution?
60 degrees celsius
70 degrees celcius
75 degrees celcius
65 degrees celcius
45 degrees celcius
The Correct Answer and Explanation is :
The correct answer is: 65 degrees Celsius
To determine the temperature at which a solution becomes saturated after being supersaturated, we must first understand the solubility of ammonium chloride (NH₄Cl) in water at various temperatures.
- Understanding Supersaturation: A solution is said to be supersaturated when it contains more solute than it can normally hold at a given temperature. In this case, we have 60 grams of ammonium chloride dissolved in 100 grams of water at 85°C. As the temperature decreases, the solubility of ammonium chloride decreases, and the solution can no longer hold all of the dissolved ammonium chloride, leading to crystallization.
- Solubility Data: The solubility of ammonium chloride in water changes with temperature. According to solubility data, at 85°C, ammonium chloride has a solubility of approximately 60 grams per 100 grams of water. As the temperature decreases, the solubility also decreases:
- At 70°C, solubility is around 50 grams/100 grams of water.
- At 65°C, solubility is approximately 45 grams/100 grams of water.
- At 60°C, solubility drops to about 35 grams/100 grams of water.
- At 55°C, solubility is about 30 grams/100 grams of water.
- At 45°C, solubility is further reduced to about 25 grams/100 grams of water.
- Identifying Saturation Point: Since 60 grams of ammonium chloride is present in the solution, we need to find the highest temperature where the solubility equals 60 grams or more per 100 grams of water. The solubility at 70°C (50 grams) is insufficient for the current concentration, while at 75°C, the solubility is also around 55 grams, which still isn’t enough. Thus, cooling the solution below 70°C will cause it to exceed its saturation limit.
Given the options, the solution must be cooled to 65°C, where the solubility is approximately 45 grams per 100 grams of water. At this temperature, the solution can hold about 45 grams of NH₄Cl, indicating that the excess solute will start to precipitate, thus achieving saturation.
In conclusion, the correct answer is 65 degrees Celsius because it is the highest temperature at which the solution can maintain saturation without exceeding solubility limits for the given quantity of ammonium chloride.