colour change of indicator 問題

天

2009-04-02 10:20 pm

我睇過好多回答, 對indicator 係end point 既 colour change 仲好混亂

1.strong acid add into strong alkali, phenolphthalein做 indicator, colour change 係 colourless to pink, colourless to red 得唔得

2.strong alkali add into strong acid, phenolphthalein做 indicatoor, colour change 點解係 red/pink to colourless 唔係 red to pink

3.weak acid add into strong alkali, phenolphthalein做 indicator, colour change 係唔係 colourless to pink?點解唔係 colourless to very pale pink?

4.strong alkali add into weak acid, phenolphthalein做 indicator, colour change 係 red to pink 定 red to colourless

我真係超混亂, 請高手解答我以上問題thx!!!

更新1:

我明左好多啦, 問多你少少野 acid 加落 alkali, methyl orange 做 indicator, colour change 係 yellow to orange? alkali 加落 acid, methyl orange 做 indicator, colour change 係 red to orange?

回答 (1)

老爺子

2009-04-03 10:32 am

✔ 最佳答案

1.
phenolphthalein 的顏色：pink (or red) in alkali, colourless in alkali, pale pink (or very pale pink) during colour change。
所以正確的答案是 from colourless to pale pink。
若果是 from colourless to pink 的話，所加入的 alkali 便是過多了。因此，很多時考試未必會接納 colourless to pink 這個答案。
是否接納 colourless to red 這個答案，我的答案是「何必冒險呢？」

2.
如第一題所述，phenolphthalein 的 equivalence point colour 是 pale pink。
當 strong alkali to acid 時（無不論是 strong acid 或 weak acid），若果你要觀察 from pink to pale pink 時，這是沒有可能的。所以通常會落多 1/4 滴（或更少）的 strong alkali，令到 solution turns colourless 才可以確定 end point。

3.
Refer to Q.1。無論是 strong acid 或 weak acid，the end point colour change is from colourless to pale pink。

4.
Refer to Q.2 and Q.3。無論是 strong acid 或 weak acid，The end point colour change is from pink to colourless.
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