iamwhoiam 發問於 科學及數學化學 · 9 年前

# 2 CHEM QUESTIONS

1.why sodium hydroxide is added to solutions containing metal ions to form insoluble hydroxide? Can potassium hydroxide be used instead?2.why strong alkalis have higher pH values than weak alkalis?

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1a). Why sodium hydroxide is added to solutions containing metal ions to form insoluble hydroxide?

Ans :

Because double decomposition occurs,

the hydroxide ion in sodium hydroxide combined with the metal ion, like Fe(II) ion, Fe(III) ion, Mg2+ ion, etc.

Then, insoluble metal hydroxide will form.

Ionic equation : M(+) (aq) + OH(-) ---> MOH (s) , where M is the metal ion.

1b) Can potassium hydroxide be used instead?

If you mean "can KOH be used instead of NaOH", the answer is YES. As potassium is more reactive than sodium, the metal ions that can be displaced by sodium can also be displaced by potassium.

But if you means "can KOH be used instead of solutions containing metal ions", the answer is NO. Because when KOH is mixed with NaOH, no reaction will take place. You will only ontain a mixture of ions, include K+, Na+ and OH-; all of them exists as aqueous form, so, no insoluble hydroxide will form.

2) Why strong alkalis have higher pH values than weak alkalis?

Ans :

I don't think that strong alkali necessarily have higher pH value than weak alkali.

For example,

pH value of 0.001M KOH(strong alkali) = 11

pH value of M ammonia solution(weak alkali) = 16

** Note that : the strength of an acid/alkali is NOT related to its pH value

** pH value depends on the concentration/molarity of the acid/alkali

黑白灰_唯物主義者_魑魅魍魎

資料來源： 黑白灰_唯物主義者_魑魅魍魎
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