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3.23 The solubility of 
Cr(NO_(3))_(3)*9H_(2)O in water is 
208g per 
100_(g) of water at 
15^(@)C. A solution of 
Cr(NO_(3))_(3)*9H_(2)O 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?

33.2323 The solubility of Cr(NO3)39H2O \mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3} \cdot 9 \mathrm{H}_{2} \mathrm{O} in water is 208 g 208 \mathrm{~g} per 100g 100_{\mathrm{g}} of water at 15C 15^{\circ} \mathrm{C} . A solution of Cr(NO3)39H2O \mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3} \cdot 9 \mathrm{H}_{2} \mathrm{O} in water at 35C 35^{\circ} \mathrm{C} is formed by dissolving 324 g 324 \mathrm{~g} in 100 g 100 \mathrm{~g} of water. When this solution is slowly cooled to 15C 15^{\circ} \mathrm{C} , no precipitate forms, (a) Is the solution that has cooled down to 15C 15^{\circ} \mathrm{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?

Full solution

Q. 33.2323 The solubility of Cr(NO3)39H2O \mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3} \cdot 9 \mathrm{H}_{2} \mathrm{O} in water is 208 g 208 \mathrm{~g} per 100g 100_{\mathrm{g}} of water at 15C 15^{\circ} \mathrm{C} . A solution of Cr(NO3)39H2O \mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3} \cdot 9 \mathrm{H}_{2} \mathrm{O} in water at 35C 35^{\circ} \mathrm{C} is formed by dissolving 324 g 324 \mathrm{~g} in 100 g 100 \mathrm{~g} of water. When this solution is slowly cooled to 15C 15^{\circ} \mathrm{C} , no precipitate forms, (a) Is the solution that has cooled down to 15C 15^{\circ} \mathrm{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?
  1. Given Solubility Data: Solubility at 15C15^\circ\text{C} is 208g208\,\text{g} per 100g100\,\text{g} of water. The solution has 324g324\,\text{g} of Cr(NO3)39H2O\text{Cr(NO}_3)_3\cdot9\text{H}_2\text{O} in 100g100\,\text{g} of water at 35C35^\circ\text{C}.
  2. Exceeding Solubility Limit: When cooled to 15C15\,^\circ\mathrm{C}, the solubility should be 208g208\,\mathrm{g}, but we have 324g324\,\mathrm{g} in 100g100\,\mathrm{g} of water, which is more than the solubility limit.
  3. Supersaturation Identified: Since the amount of solute is greater than the solubility limit at 15C15^\circ\text{C}, the solution is supersaturated.
  4. Nucleation Initiated: Scratching the glass vessel likely initiated nucleation, causing the supersaturated solution to begin crystallizing.
  5. Calculate Expected Crystal Mass: To find the mass of crystals expected to form, subtract the solubility limit at 15C15^\circ\text{C} from the initial amount of solute.
  6. Subtracting Solubility Limit: Initial amount of solute is 324g324\text{g}. Solubility limit at 15C15^\circ\text{C} is 208g208\text{g}. Expected mass of crystals = 324g208g324\text{g} - 208\text{g}.
  7. Final Crystal Mass Calculation: Expected mass of crystals = 324g208g=116g324\text{g} - 208\text{g} = 116\text{g}.

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