U3.10 · SolubilityPractice · 溶解度

Per 100 g of water, 60.2 − 37.2 = 23.0 g crystallizes. Here there is only 50.0 g of water, half of 100 g, so 23.0 × 50.0 ÷ 100 = 11.5 g crystallizes. Solubility is given per 100 g of water, so scale it when the water isn’t 100 g.

Each of these alcohols has an –OH group that hydrogen-bonds with water, but the rest of the molecule is a nonpolar hydrocarbon chain. Pentanol, $\ce{CH3(CH2)4OH}$, has the longest chain and the biggest nonpolar part, which disrupts many hydrogen bonds between water molecules without making up for them, so it is least soluble. Methanol and ethanol mix with water in all proportions, and ethylene glycol, with two –OH groups, does too.

Compare them at each temperature: 10 °C, $\ce{KNO3}$ 20.9 and NaCl 35.8; 20 °C, $\ce{KNO3}$ 31.6 and NaCl 36.0; 30 °C, $\ce{KNO3}$ 45.8 and NaCl 36.3; 40 °C, $\ce{KNO3}$ 63.9 and NaCl 36.6. At 20 °C $\ce{KNO3}$ is still below NaCl, and by 30 °C it is above, so the two solubility curves cross between 20 and 30 °C. $\ce{KNO3}$’s solubility changes a lot with temperature; NaCl’s barely changes.

100 g of water can hold at most 40.0 g of KCl, so the solution becomes saturated and 50.0 − 40.0 = 10.0 g stays at the bottom. The solution is saturated and contains 40.0 g of dissolved KCl, not 50.0 g.

Most solids get more soluble when heated, but $\ce{Ca(OH)2}$ does the opposite. The 0.165 g dissolved at 20 °C (per 100 g of water) is more than the 0.094 g that 80 °C can hold, so about 0.071 g comes out of solution and it turns cloudy. Don’t assume heating always dissolves more.