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一、明确求解原则——“先定后算”“先定后算”是指在运用溶质质量分数的求解公式进行解题时,必须先把“溶液中的溶质是什么,是多少”、“溶液是什么,是多少”等问题搞清后才能代入公式进行求解,否则就会出错,其中,“先定”是前提,是关键。例1将30g A物质加入到70g水中,求所得溶液的溶质质量分数。解析:有些同学见题后就立刻回答是30%,对吗?根据“先定后算”的求解原则,应先定下来溶质、溶液各是什么、各是多少。溶液是A的溶液吗,质量是100g吗?溶质是A吗,是30g吗?简单分析如下:
First, the principle of a clear solution - “first set after the count ” “first set after the count ” refers to the use of solute mass fraction formula to solve the problem, you must first “solute in the solution is what, How many ”“, ”What is the solution, is the number “ and other issues can only be calculated into the formula to solve, otherwise it will be an error, where ”first set “ is the premise is the key. Example 1 30 g of substance A was added to 70 g of water to determine the solute mass fraction of the resulting solution. Analysis: Some students answer the question immediately after the answer is 30%, right? According to ”first set after the count " principle of the solution, you should set the solute, the solution is what, each is how much. Is the solution A solution, the quality is 100g? Solute is A, is 30g? Simple analysis is as follows: