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例:配制500毫升30％的硫酸溶液(比重为1.22/克毫升)需要98％的浓硫酸(比重为1.84克/毫升)和水各多少毫升? 这道题有部分同学及少数教师的解法如下: 所需的浓硫酸毫升数例:配制500毫升30％的硫酸溶液(比重为1.22/克毫升)需要98％的浓硫酸(比重为1.84克/毫升)和水各多少毫升? 这道题有部分同学及少数教师的解法如下: 所需的浓硫酸毫升数=(500毫升×1.22克/毫升×30％)/(8.4 克/毫升×98％)=101.7毫升。 所需水的毫升数=500毫升-101.7毫升=398.3毫升。 上述水的体积的解法是错误的。正确的解法应为: 所需水的毫升数=(水的质量)/(水的密度)=500毫升×1.22克/毫升-101.7毫升×1.84克/毫升 1克/毫升=422.9毫升。 错误的原因,是对两种分子结构相差较大的液体互溶后总体积减少没有理解。 我们知道,任何物质分子与分子之间都是有间隔的,而不同物质分子间间隔的大小也不完全一样。分子结构相差较大的物质混溶后,一种物质的部分分子钻到另一种物质分子的间隙中去,这样,总体积自然就减小了。就象一斗芝麻与一斗大豆混和均匀总体积不等于两斗一样。许多物质与水互溶,由于产生缔合、或形成水合分子(水合离子),使分子间间隔缩小而引起总体积的减少。
Example: Formulation of 500 ml of 30% sulfuric acid solution (specific gravity of 1.22/g ml) requires 98% concentrated sulfuric acid (specific gravity 1.84 g/ml) and how many milliliters of water each? This part of the solution for some students and a few teachers is as follows : Concentrated milliliters of sulfuric acid required Example: Preparation of 500 ml of 30% sulfuric acid solution (specific gravity of 1.22/g ml) requires 98% concentrated sulfuric acid (specific gravity of 1.84 g/ml) and the number of milliliters of water each? The solutions of some students and a few teachers are as follows: The required milliliter of concentrated sulfuric acid = (500 ml × 1.22 g/ml × 30%)/(8.4 g/ml × 98%) = 101.7 ml. The required water ml = 500 ml - 101.7 ml = 398.3 ml. The solution of the above water volume is wrong. The correct solution should be: Milliliters of required water = (mass of water)/(density of water) = 500 ml × 1.22 g/ml - 101.7 ml × 1.84 g/ml 1 g/ml = 422.9 ml. The reason for the error is that there is no understanding of the decrease in the total volume of the liquid that has a large difference between the two molecular structures. We know that there is a gap between any material molecule and molecule, and the size of the gap between different material molecules is not exactly the same. After a material with a large difference in molecular structure is miscible, a part of a molecule of a substance is drilled into a gap between molecules of another substance, so that the total volume is naturally reduced. Just like a bucket of sesame seeds mixed with a bucket of soybeans, the total volume is not equal to two buckets. Many substances are miscible with water, resulting in a decrease in the total volume due to association or the formation of hydrated molecules (hydrated ions) that reduce the intermolecular spacing.