alkali

Question

35.When an allkali is added to Zn(NO3)2 solution,a white ppt is formed.This show that
A. the alkali contains OH-
B. the allkali contains metallic ions
C. ZnO is insoluble in water
D. the alkali is strong electrolyte
Why the answer is A but NOT C?

36. Which of the following hydroxides would react with both acid and alkali to give a salt and water
A Zinc hydroxide
B copper(II) hydroxide
C iron(II) hydroxide
D calcium hydroxide

Answer 1

35.
The precipitate is Zn(OH)2, but not ZnO.
Equation : Zn2+(aq) + 2OH-(aq) → Zn(OH)2(s)

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36.
The answer is A, because Zn(OH)2 is amphoteric (兩性的) which can neutralize both strong acids and strong alkalis.
Zn(OH)2(s) + 2H+(aq) → Zn2+(aq) + 2H2O(l)
Zn(OH)2(s) + 2OH-(aq) → [Zn(OH4)]2-(aq)
The second reaction does not give water as a product. However, hydroxide ions are consumed and a salt, [Zn(OH4)]2-(aq), is formed. The reaction is also classified as “neutralization”.