Question

A gas obtained by the addition of dilute \[{H_2}S{O_4}\] to a mixture which turns lead acetate paper black. It is:
(A) \[S{O_2}\]
(B) \[C{O_2}\]
(C) \[{H_2}S\]
(D) \[N{O_2}\]

Answer 1

Hint: In order to turn the lead acetate paper black, the gas should react with lead acetate in a way that it forms a compound that is black in color so that the paper would also get black in color. Sulphides of some metals like copper and lead are black in color.

Complete step by step answer:
Here, we are asked which gas will turn lead acetate paper black that can be produced by the reaction of a mixture with dilute sulfuric acid.
We will confirm which gas will turn lead acetate paper black and then will confirm the reaction with sulfuric acid and mixture.
- \[{H_2}S\] gas reacts with lead acetate to produce Lead sulphide and acetic acid. Lead sulphide is black in colour. So, the paper in which lead acetate is present, will turn black as the formation of lead sulphide will occur there. The reaction can be given as below.
\[Pb{(C{H_3}COO)_2} + {H_2}{S_{(g)}} \to PbS + 2C{H_3}COOH\]
So, we can say that hydrogen sulphide gas will turn the lead acetate paper black.
- Sulfur dioxide gas cannot react with lead acetate, hence it will not change the color of the lead acetate paper.
- Nitrogen dioxide can also not react with lead acetate solution, so it will also not be able to change the color of the lead acetate paper.
- Carbon dioxide reacts with lead acetate to give lead carbonate but lead carbonate is white in color. So, it will not be able to turn lead acetate paper black. The reaction is given below.
\[C{O_2} + Pb{(C{H_3}COO)_2} \to PbC{O_3} + C{H_3}CO - O - COC{H_3}\]

Thus, only Hydrogen sulphide gas can turn lead acetate paper black which can be confirmed by the following reaction.
\[FeS + {H_2}S{O_4} \to {H_2}S + FeS{O_4}\]
In the given reaction, we can see that as sulfuric acid is added to ferrous sulphide containing vessel, hydrogen sulphide gets formed and as we bring lead acetate paper close to the vessel, the \[{H_2}S\] gas there will react with lead acetate on the paper and will form lead sulfide there which will turn lead acetate paper black.

So, the correct answer is (C) \[{H_2}S\].


Note:

Remember that it does not mean that if the reaction of gas and lead acetate occurs, it will always turn lead acetate paper black. It should form a compound that is black in color to turn the paper black-that is the reason why carbon dioxide does not turn lead acetate paper black even though it can react with lead acetate.