Question

When gaseous hydrocarbons are completely burnt in an eudiometer tube with excess of ${{\text{O}}_2}$ and after cooling there is a contraction in the volume of gases.
Reason:
The volume of ${{\text{H}}_2}{\text{O}}$ is not considered.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Both Assertion and Reason are incorrect.

Answer 1

Hint: When a hydrocarbon is burnt in the presence of oxygen, it is called combustion reaction. It produces carbon dioxide and water molecules. When it is cooled, there is a volume contraction of gases.

Complete step by step answer:
Hydrocarbons are burnt in the presence of excess oxygen. The balanced equation of combustion reaction is given below:
${{\text{C}}_{\text{x}}}{{\text{H}}_{\text{y}}} + \left( {{\text{x}} + \dfrac{{\text{y}}}{4}} \right){{\text{O}}_2} \to {\text{xC}}{{\text{O}}_2} + \dfrac{{\text{y}}}{4}{{\text{H}}_2}{\text{O}}$
${\text{x}}$ tells the number of carbon atoms.
${\text{y}}$ tells the number of hydrogen atoms.
Using volume ratio,
Volume of oxygen used$ = $$\left( {{\text{x}} + \dfrac{{\text{y}}}{4}} \right){\text{c}}{{\text{m}}^3}$
Volume of carbon dioxide produced $ = $${\text{xc}}{{\text{m}}^3}$
Volume of water as liquid $ = $ $0{\text{c}}{{\text{m}}^3}$
Volume of hydrocarbon used for burning $ = $$1{\text{c}}{{\text{m}}^3}$
Contraction in the volume, $\Delta {\text{V = }}{{\text{V}}_{\text{R}}} - {{\text{V}}_{\text{P}}}$
Substituting the values, we get
$\Delta {\text{V}} = \left[ {1 + \left( {{\text{x}} + \dfrac{{\text{y}}}{4}} \right) - {\text{x}}} \right]$
The volume of ${{\text{H}}_2}{\text{O}}$ is not considered because of cooling water vapor condenses. Thus the variables of water molecules are not considered.
Hence, Option A is the correct option.

Additional information:
From the value of contraction volume, the value of ${\text{x}}$ and ${\text{y}}$ is calculated and then the molecular formula is obtained.

Note:
Using this volume contraction, molecular formula can be calculated, thereby molecular mass can be calculated. For this the volume of water molecules is not considered. At room temperature, water is actually a liquid, thereby the volume becomes negligible.