Question

Sodium hydroxide is added drop wise until in excess to a solution of zinc sulphate. Which of the following observations can be made?
A) White precipitate is formed which is soluble in excess of sodium hydroxide.
B) White precipitate is formed which is insoluble in excess of sodium hydroxide.
C) Black precipitate is formed which is soluble in excess of sodium hydroxide.
D) Black precipitate is formed which is insoluble in excess of sodium hydroxide.

Answer 1

To solve this we must know the reaction that occurs when sodium hydroxide reacts with zinc sulphate. The precipitate is formed in the reaction. We must know the colour of the precipitate and its solubility in sodium hydroxide.

Complete answer:
We are told that sodium hydroxide is added drop wise until in excess to a solution of zinc sulphate.
The molecular formula for sodium hydroxide is ${\text{NaOH}}$ and the molecular formula for zinc sulphate is ${\text{ZnS}}{{\text{O}}_{\text{4}}}$.
Thus, the reaction of sodium hydroxide with zinc sulphate is as follows:
${\text{ZnS}}{{\text{O}}_{\text{4}}} + {\text{2NaOH}} \to {\text{Zn}}{\left( {{\text{OH}}} \right)_2} + {\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4}$
Thus, when sodium hydroxide reacts with zinc sulphate a precipitate of zinc hydroxide $\left( {{\text{Zn}}{{\left( {{\text{OH}}} \right)}_2}} \right)$ is formed. The zinc hydroxide precipitate is white in colour.
When excess sodium hydroxide is added to a solution of zinc sulphate the precipitate of zinc hydroxide which is formed reacts with sodium hydroxide.
The reaction of white precipitate of zinc hydroxide with sodium hydroxide is as follows:

${\text{Zn}}{\left( {{\text{OH}}} \right)_2} + {\text{NaOH}} \to {\text{N}}{{\text{a}}_2}{\text{Zn}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}}$

Thus, when white precipitate of zinc hydroxide reacts with sodium hydroxide a soluble salt of sodium and zinc $\left( {{\text{N}}{{\text{a}}_2}{\text{Zn}}{{\text{O}}_2}} \right)$ is formed. Thus, the white precipitate of zinc hydroxide is soluble in sodium hydroxide.
Thus, when sodium hydroxide is added drop wise until in excess to a solution of zinc sulphate it is observed that white precipitate is formed which is soluble in excess of sodium hydroxide.

Thus, the correct option is (A) colourless.

Note: When two compounds exchange parts to form two different compounds, the reaction is known as a double displacement reaction. Thus, the reaction of sodium hydroxide with zinc sulphate is a double displacement reaction.