CHEMISTRY
JAMB 2006 - Question 9
Chemistry 2006 JAMB Past Questions - Question 9: The ion that will give a white precipitate with aqueous NaOH and soluble in excess of the base is
Choose the correct answers from the options given.
A:
B:
C:
D:
Correct Answer
C
Explanation
The ion that will give a white precipitate with aqueous NaOH and is soluble in excess of the base is zinc ion (Zn²+). When zinc ion reacts with aqueous sodium hydroxide (NaOH), it forms a white precipitate of zinc hydroxide (Zn(OH)₂). However, this precipitate is soluble in excess sodium hydroxide, forming a colorless solution of zincate ions:
Zn²+ + 2NaOH → Zn(OH)₂ (white precipitate)
Zn(OH)₂ + 2NaOH → [Zn(OH)₄]²⁻ (soluble complex)
In contrast, calcium ion (Ca²+), magnesium ion (Mg²+), and copper ion (Cu²+) form insoluble hydroxide precipitates with sodium hydroxide that do not dissolve in excess base.
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