一元弱酸溶液氢离子浓度近似计算的课程新设计
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摘要
通常情况下,一元弱酸HA溶液氢离子浓度可按近似式■估算。不同于现有教科书,本文以新的方式导出上述公式,并用数学方法推导出了[H~+]_(AV1)和[H~+]_(AV2)近似式的使用条件。(1)溶液中[H~+][OH-]时,可忽略水的离解,此时可用[H~+]_(AV1)估算溶液pH。[H~+]_(AV1)与氢离子准确浓度[H~+]T相对误差不大于±5%时,需要满足两条件c>6.0×10~(-6)mol·L~(-1)和c K_a≥10K_w;(2)用[H~+]_(AV2)估算溶液pH的条件为[H~+]■[OH-]和[HA]≈c,即同时忽略水的离解和忽略HA的离解。[H~+]_(AV2)与[H~+]T相对误差不大于±5%时,要求满足c K_a≥10K_w和c≥105K_a。本文首次用数学方法推导出c K_a≥10K_w或c≥105K_a等条件,与现教科书有关内容比较,具有判据严谨、边界条件清晰、学生容易理解等优点,对提高教学效果具有积极意义。
Among the mainstream analytical chemistry textbooks,the hydrogen ion concentration of a monoprotic weak acid solution can be estimated by approximation formulas■.In this paper,the two above formulas are derived from a new and easy understanding way.(1) If [H~+]■[OH-],[H~+]_(AV1) can be used to estimate the pH of the solution.If the relative error between [H~+]_(AV1) and the accurate hydrogen ion concentration [H~+]Tis within ±5%,then both c>6×10~(-6) mol·L~(-1) and c K_a≥10 K_w are required.(2) When use [H~+]_(AV2) to estimate the pH of the solution,it needs two conditions:[H~+]■[OH-]and c≈[HA].If the relative error between [H~+]_(AV2) and [H~+]Tis less than ±5%,both c K_a≥10 K_w and c≥105 K_a are required.The conclusions obtained in this study are more accurate and reliable,the physical meanings of the approximation formulas are clear.Therefore,it is easy for the students to understand and master the related approximate calculation correctly.
Among the mainstream analytical chemistry textbooks,the hydrogen ion concentration of a monoprotic weak acid solution can be estimated by approximation formulas■.In this paper,the two above formulas are derived from a new and easy understanding way.(1) If [H~+]■[OH-],[H~+]_(AV1) can be used to estimate the pH of the solution.If the relative error between [H~+]_(AV1) and the accurate hydrogen ion concentration [H~+]Tis within ±5%,then both c>6×10~(-6) mol·L~(-1) and c K_a≥10 K_w are required.(2) When use [H~+]_(AV2) to estimate the pH of the solution,it needs two conditions:[H~+]■[OH-]and c≈[HA].If the relative error between [H~+]_(AV2) and [H~+]Tis less than ±5%,both c K_a≥10 K_w and c≥105 K_a are required.The conclusions obtained in this study are more accurate and reliable,the physical meanings of the approximation formulas are clear.Therefore,it is easy for the students to understand and master the related approximate calculation correctly.
引文
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[2]武汉大学.分析化学(第6版).北京:高等教育出版社,2016:120~122.
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[2]武汉大学.分析化学(第6版).北京:高等教育出版社,2016:120~122.
[3]陈国松,张莉莉.分析化学.南京:南京大学出版社,2014:116~117.
[4]胡乃非,欧阳津,晋卫军等.分析化学(第3版).北京:高等教育出版社,2010,58~60.
[5]张耀东,蔡亚楠,刘爱华.大学化学,2012,27(6):80~82.