Which ion reacts with aqueous sodium hydroxide to give a precipitate that dissolves in an excess of sodium hydroxide?
A Ca2+
B Fe2+
C Fe3+
D Pb2+
Pls explain how to get the ans, thks!
All form a hydroxide insoluble in water. Pb^2+ is the only one to form Pb(OH)2 which dissolves in an excess of NaOH by forming a complex ion of [Pb(OH)4]^2-
To determine which ion reacts with aqueous sodium hydroxide to give a precipitate that dissolves in an excess of sodium hydroxide, we need to understand the solubility rules for ionic compounds.
The solubility rules state that most hydroxides are insoluble in water, except for hydroxides of Group 1 metals (such as sodium) and some of the heavier Group 2 metals. However, when an excess of sodium hydroxide is added, the hydroxide ions can react with the metal ions to form soluble complexes.
To find the answer, we need to examine the solubility of the hydroxides of calcium (Ca2+), iron (Fe2+), iron (Fe3+), and lead (Pb2+).
A. Ca2+: Calcium hydroxide (Ca(OH)2) is sparingly soluble in water, forming a white precipitate. However, this precipitate does not dissolve in an excess of sodium hydroxide. Therefore, option A is not the correct answer.
B. Fe2+: Iron(II) hydroxide (Fe(OH)2) is also sparingly soluble in water, forming a green precipitate. However, when excess sodium hydroxide is added, the precipitate does not dissolve. Therefore, option B is not the correct answer.
C. Fe3+: Iron(III) hydroxide (Fe(OH)3) is insoluble in water, forming a reddish-brown precipitate. However, when excess sodium hydroxide is added, the precipitate does dissolve to form a brownish solution. Therefore, option C is the correct answer.
D. Pb2+: Lead(II) hydroxide (Pb(OH)2) is also insoluble in water, forming a white precipitate. However, when excess sodium hydroxide is added, the precipitate does not dissolve. Therefore, option D is not the correct answer.
In summary, the correct answer is option C: Fe3+. Iron(III) ions react with aqueous sodium hydroxide to give a precipitate of iron(III) hydroxide, which dissolves in an excess of sodium hydroxide.