在20℃时，将100克溶质质量分数为20％的氯化钠溶液蒸发掉20克水，温度再恢复到20℃，求析出氯化钠晶体的质量?(20℃时氯化钠的溶解度为36.0克)
解：20℃时氯化钠的溶解度为36.0克，即100克水中能溶解36克
设蒸发掉20g水后科溶解氯化钠的质量为X，即36/100=X/60，解得X=21.6g
因为溶液原来就只有20克氯化钠，所以不会析出晶体。
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