Question

A $0.92$ g of organic compound was analysed by combustion method. The mass of the U-tube increased by $1.08{\text{ g}}$ . What is the percentage of hydrogen in the compound?
(A) $13.04\%$
(B) $52.17\%$
(C) $65.21\%$
(D) $11.30\%$

Answer 1

First of all let’s understand the combustion reaction and what all products are formed out of the combustion reaction. So, the organic compounds react with oxygen to produce a large amount of energy, carbon- dioxide and water.

Complete answer:
Combustion is an exothermic reaction where lots of heat is produced during the reaction.
The amount of carbon, oxygen and hydrogen present in the starting of the reaction, that is the amount of reactants can be found easily by calculating the amount of products formed out of the combustion reaction.
This is possible because the combustion reaction is a pure and clean reaction as it leaves no residue behind it.
So, let’s calculating our desired percentage of hydrogen in the compound:
Increase in mass of U-tube $= 1.08{\text{ g}}$
So, we can say that the mass of water formed is $= 1.08{\text{ g}}$
Now, we will use unitary method to solve this equation:
Since, in $18g$ of water $= 2g$ of $(H)$ hydrogen is present
So, in $1.08{\text{ g}}$ of water $= \dfrac{2}{{18}} \times 1.08{\text{ g}}$ of hydrogen will be present
Now, to calculate the percentage of hydrogen, we will divide this value with the mass of organic compounds. So we will get:
Percentage of hydrogen $\%$ $= \dfrac{2}{{18}} \times \dfrac{{1.08{\text{ }}}}{{0.92}} \times 100$
$= 13.04\%$
Therefore the correct answer is option (A) i.e. $= 13.04\%$ .

Note:
Here in calculating the answer, we have used a simple stoichiometric concept and linked it with the unitary method. This is the easiest and quick way to calculate the answer. Also, note that as there are two molecules of hydrogen in the $18g$ of water, that’s why we have taken $2g$ of hydrogen.