初中化学第二单元,有这样一个问题.即:根据化学式求算物质中某元素质量分数.课本上介绍的仅仅是纯物质中某元素质量分数的计算.对于同学们来说,在理解式量求算的基础上掌握这种计算并不费劲.而在现实生活中绝对纯净的物质是没有的,我们在解决实际问题时常遇到的是一些不纯物质(含杂质)中某元素的 The second unit of junior middle school chemistry has such a problem. That is: According to the chemical formula, the mass fraction of an element in a material is calculated. The textbook introduces only the calculation of the mass fraction of an element in the pure material. For the students, the quantity is understood in the understanding. It is not difficult to master such calculations on the basis of calculation. However, in the real life, absolutely pure substances are not available. We often encounter certain elements of impure substances (including impurities) in solving practical problems.