Question

What is the gas liberated when an alkaline formaldehyde solution is reacted with hydrogen peroxide in the presence of pyrogallol?
A.\[{{H}_{2}}\]
B.\[{{O}_{2}}\]
C.\[C{{H}_{4}}\]
D.\[C{{O}_{2}}\]

Answer 1

Hint: ${{H}_{2}}{{O}_{2}}$ acts as an oxidizing reagent. It oxidizes formaldehyde (HCHO) in either an alkaline or acidic medium. In the acidic medium, formaldehyde is mineralized to carbon dioxide. In the alkaline medium, formaldehyde is converted to sodium formate.

Complete answer:
Hydrogen peroxide reacts with formaldehyde in two mediums.
We have to see the reaction that occurs between the alkaline formaldehyde and hydrogen peroxide. This reaction occurs in the presence of pyrogallol. The reaction also includes sodium hydroxide (NaOH).
When formaldehyde reacts with hydrogen peroxide in the presence of pyrogallol we get sodium formate, water, and hydrogen gas is liberated.
The reaction between the formaldehyde and hydrogen peroxide in an alkaline solution is as follows:
\[2HCHO+2NaOH+{{H}_{2}}{{O}_{2}}\to 2HCOONa+2{{H}_{2}}O+{{H}_{2}}\uparrow \]
Therefore, the gas liberated when an alkaline formaldehyde solution is reacted with hydrogen peroxide in the presence of pyrogallol is ${{H}_{2}}$.

The correct answer is Option A- ${{H}_{2}}$ .

Additional information:
The reaction between Hydrogen peroxide and acidic formaldehyde is a two-step process.
In the first step, formaldehyde is converted to formic acid and in the second step, it is mineralized to carbon dioxide.
Let’s see both the reactions,
Step 1: \[\begin{array}{*{35}{l}}
   HCHO\text{ }+\text{ }{{H}_{2}}{{O}_{2}}\text{ }\to \text{ }HCOOH\text{ + }{{\text{H}}_{2}} \\
\end{array}O\]
Step 2: \[\begin{array}{*{35}{l}}
   HCOOH\text{ }+\text{ }{{H}_{2}}{{O}_{2}}\text{ }\to \text{ + 2}{{\text{H}}_{2}} \\
\end{array}O\text{ + C}{{\text{O}}_{^{2}}}\]
Thus, the gas liberated when acetic formaldehyde solution is reacted with hydrogen peroxide is $C{{O}_{2}}$.

Note:
Pyrogallol is used in the reaction to absorb the oxygen in the reaction. It is also used to determine the oxygen content in the mixture of gases. However, alkaline pyrogallol absorbs water as well as oxygen. So, in the reaction between alkaline formaldehyde and hydrogen peroxide pyrogallol is used to remove the water formed.