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题目:怎样除去NaCl中含有的少量MgCl_2。 分析:解除杂题时应做到: 1.选用试剂要与杂质充分反应; 2.不能引入新的杂质; 3.除杂后要将所得物质恢复为原状态。 解除杂题的关键是如何选用所加试剂。通常杂质与主要成分间含有共同成分,可将其转化为主要成分,而将不同成分除去,其用到的知识主要涉及复分解反应条件及物质的溶解性等,运用填空法可一目了然。如此题可写成: MgCl_2+Na□—NaCl+Mg□↓。 观察与Mg~(2+)可形成的不溶物是Mg(OH)_2,再考虑OH~-与Na~+能组成可溶物NaOH,反应可行,所以用
Title: How to remove a small amount of MgCl 2 contained in NaCl. Analysis: When solving the miscellaneous questions, it should be done as follows: 1. Select reagents to fully react with impurities; 2. Do not introduce new impurities; 3. After the impurities are removed, the obtained substances shall be restored to the original state. The key to solving the problem is how to use the added reagents. Usually impurities and main components contain common components, which can be converted into the main components, while the removal of different components, the knowledge used mainly relates to metathesis reaction conditions and the solubility of the material, etc. The use of blank method can be clear at a glance. This question can be written as: MgCl_2+Na□—NaCl+Mg□↓. Observe that the insoluble matter formed with Mg 2+ is Mg(OH) 2 , and consider that OH~- and Na~+ can form soluble NaOH and the reaction is feasible.