3.23 The solubility of Cr(NO3)3⋅9H2O in water is 208g per 100g of water at 15∘C. A solution of Cr(NO3)3⋅9H2O in water at 35∘C is formed by dissolving 324g in 100g of water. When this solution is slowly cooled to 15∘C, no precipitate forms, (a) Is the solution that has cooled down to 15∘C unsaturated, saturated, or supersaturated? (b) You take a metal spatula and scratch the side of the glass vessel that contains this cooled solution, and crystals start to appear. What has just happened? (c) At equilibrium, what mass of crystals do you expect to form?
Q. 3.23 The solubility of Cr(NO3)3⋅9H2O in water is 208g per 100g of water at 15∘C. A solution of Cr(NO3)3⋅9H2O in water at 35∘C is formed by dissolving 324g in 100g of water. When this solution is slowly cooled to 15∘C, no precipitate forms, (a) Is the solution that has cooled down to 15∘C unsaturated, saturated, or supersaturated? (b) You take a metal spatula and scratch the side of the glass vessel that contains this cooled solution, and crystals start to appear. What has just happened? (c) At equilibrium, what mass of crystals do you expect to form?
Given Solubility Data: Solubility at 15∘C is 208g per 100g of water. The solution has 324g of Cr(NO3)3⋅9H2O in 100g of water at 35∘C.
Exceeding Solubility Limit: When cooled to 15∘C, the solubility should be 208g, but we have 324g in 100g of water, which is more than the solubility limit.
Supersaturation Identified: Since the amount of solute is greater than the solubility limit at 15∘C, the solution is supersaturated.
Nucleation Initiated: Scratching the glass vessel likely initiated nucleation, causing the supersaturated solution to begin crystallizing.
Calculate Expected Crystal Mass: To find the mass of crystals expected to form, subtract the solubility limit at 15∘C from the initial amount of solute.
Subtracting Solubility Limit: Initial amount of solute is 324g. Solubility limit at 15∘C is 208g. Expected mass of crystals = 324g−208g.
Final Crystal Mass Calculation: Expected mass of crystals = 324g−208g=116g.
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