Question

Which of the following salt gives nitrogen dioxide gas on heating?
a.) $Pb{(N{O}_{3})}_{2}$
b.) $ZnS{O}_{4}.7{H}_{2}O$
c.) $NaHS{O}_{4}$
d.) $CuS{O}_{4}.5{H}_{2}O$

Answer 1

Hint: On heating a solution, if the gas evolved is brown in colour, then the gas evolved is of nitrogen component and it is nitrogen dioxide. It is an acidic gas and turns moist blue litmus paper red. The chemical formula of nitrogen dioxide is $N{O}_{2}$.

Complete step by step solution:
Nitrogen dioxide is an acidic gas which produces an acidic solution in water. It is one of the strong acidic gases in chemistry. It reacts with water to give nitrous acid and nitric acid. The chemical formula of nitrogen dioxide is $N{O}_{2}$.
Let us now see the reaction involved when the compounds given are heated.

$Pb{(N{O}_{3})}_{2}$: When $Pb{(N{O}_{3})}_{2}$ is heated it decomposes to liberate lead monoxide, nitrogen dioxide and oxygen gas. The gas evolved is reddish brown in color which shows the presence of nitrogen dioxide gas.
$2Pb{ (N{ O }_{ 3 }) }_{ 2 }\quad \longrightarrow \quad 2PbO\quad +\quad 4N{ O }_{ 2 }\quad +\quad { O }_{ 2 }$

$ZnS{O}_{4}.7{H}_{2}O$: When $ZnS{O}_{4}.7{H}_{2}O$ is heated, it emits toxic fumes of zinc oxide and sulphur oxides and no nitrogen dioxide gas is evolved.

$NaHS{O}_{4}$: When $NaHS{O}_{4}$ is heated, it forms ${Na}_{2}S{O}_{4}$ along with ${H}_{2}O$ and ${O}_{2}$ But no nitrogen dioxide gas is evolved.
$2NaHS{ O }_{ 4 }\quad \longrightarrow \quad { Na }_{ 2 }S{ O }_{ 4 }\quad +\quad { H }_{ 2 }O\quad +\quad S{ O }_{ 3 }$

$CuS{O}_{4}.5{H}_{2}O$: When $CuS{O}_{4}.5{H}_{2}O$ is heated, it decomposes into dehydrate form and given out water.
$CuS{ O }_{ 4 }.{ 5H }_{ 2 }O\quad \longrightarrow \quad CuS{ O }_{ 4 }\quad +\quad 5{ H }_{ 2 }O$

Thus, from the above reactions we can see that only $Pb{(N{O}_{3})}_{2}$ gives out $N{O}_{2}$ gas when it is heated. Hence, option (a) is the correct answer.

Note: All the nitrate salts when heated form nitrogen dioxide gas along with oxygen. Only sodium nitrate and potassium nitrate decomposes to liberate only oxygen gas. Since, nitrogen dioxide is liberated when $Pb{(N{O}_{3})}_{2}$ is heated, it is used in pyrotechnics such as fireworks.