Solubility indicates how much of a solid or gas can dissolve. Recrystallization is a purification method that uses temperature dependence.
Basic Knowledge
The solubility of a solid is the maximum mass of solute that dissolves in 100 g of water [g/100g water]. The solubility of many salts (KNO3, NaNO3) increases as temperature ↑. As exceptions, NaCl has little temperature dependence, while Ce2(SO4)3 decreases as temperature ↑. The solubility of gases follows Henry's law and increases as temperature ↓ and partial pressure ↑. At constant temperature, the mol (mass) of dissolved gas is proportional to its partial pressure, but the volume at that partial pressure is independent of partial pressure.
Saturated solution (a solution containing as much solute as can dissolve at that temperature)
Solubility (maximum mass of solute that dissolves in 100 g of water)
Supersaturation (metastable state exceeding saturation)
Henry's law (amount of dissolved gas ∝ partial pressure)
Recrystallization (purification method involving dissolution at high temperature followed by precipitation during cooling)
Water of crystallization (water in a crystal, such as CuSO4·5H2O)
Further Study (Principles and Applications)
Recrystallization is a classic purification method that uses differences in solubility. KNO3 has a solubility of 22 g at 10℃ and 110 g at 60℃, a difference of approximately five times, making it suitable for recrystallization. For crystals containing water of crystallization (CuSO4·5H2O), a frequent mistake is to add the water of crystallization to the solvent water and calculate on an anhydrous basis. Carbonated drinks are made by dissolving CO2 under high pressure and are a familiar example of Henry's law.
- Solubility of solids: generally increases as temperature ↑
- Solubility of gases: increases as temperature ↓ and pressure ↑
- Henry's law: dissolved mol ∝ partial pressure
- At the same temperature and partial pressure, volume is constant
- Recrystallization = using differences in solubility
- Crystals containing water of crystallization require attention
- Supersaturation is metastable and crystallizes when disturbed
Important Notes (Common Confusions and Frequent Mistakes)
① Solubility is based on “100 g of water” (not 100 g of solution). ② In Henry's law, moles are proportional to partial pressure, while volume (at that partial pressure) is unchanged. ③ Include water of crystallization in the solvent water when calculating. ④ Supersaturation is metastable.
Practice
- A saturated solution of KNO3 formed by dissolving it in 100 g of water at 60℃ (110 g/100g water) is cooled to 10℃ (22 g/100g water). How many grams precipitate?
- When 25 g of CuSO4·5H2O is dissolved in 75 g of water, find the mass percent concentration of CuSO4 (M(CuSO4)=160, M(H2O)=18).
- At 0℃ and 1.0×105 Pa, 50 mL of O2 (converted to STP) dissolves in 1 L of water. At the same temperature and a partial pressure of 2.0×105 Pa, find the volume of O2 dissolved in 1 L of water, expressed at STP.