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中学化学教学大纲说明里指出:“初中学生应该掌握下列计算方面的技能:(1)根据分子式计算物质的分子量,化合物里各种元素的重量比和重量百分比。(2)根据化学方程式对反应物和生成物的量作简单计算。(3)计算溶质和溶剂的用量来制备一定量的一定百分比浓度的溶液。”在教学中发现学生对于化学计算有下列缺陷。(1)算错分子量的原因是对分子式前面的系数所表示的概念模糊。虽然,大多数学生会抽象地背诵“系数是表示整个分子的个数的”,但是竟把2H_2O 的重量算成2×2+16=20。(2)不善于运用定组成定律。如:使72克氧气完全化合成水,须用多少克氢气?他们不是根据氢、氧化合成水时的重量比固定是:1:8这一简单数据入手。1:8=72:x而是列化学方程式求解2H_2+O_2(?)2H_2O4 32x 72
According to the chemistry instruction syllabus of junior high school, “Junior high school students should master the following computational skills: (1) Calculate the molecular weight of the substance, the weight ratio of the various elements in the compound, and the weight percentage according to the formula. (2) According to the chemical equation, And the amount of product to make a simple calculation. (3) Calculate the amount of solute and solvent to prepare a certain amount of concentration of a certain percentage of the solution. ”In the teaching of students found that the following shortcomings. (1) The reason for calculating the molecular weight is to obscure the concept represented by the coefficient in front of the numerator. Although most students abstractly recite “coefficient is the number of the whole molecule,” the weight of 2H_2O is calculated as 2 × 2 + 16 = 20. (2) not good at using the law of composition. Such as: 72 grams of oxygen to complete the synthesis of water, how much need to use grams of hydrogen? They are not based on hydrogen, the weight ratio of synthetic water oxidation is fixed: 1: 8 this simple data to start. 1: 8 = 72: x but instead of the chemical equation solving 2H_2 + O_2 (?) 2H_2O4 32x 72