Question

The vapour density of ethyl alcohol vapour is:
A.46
B.23
C.92
D.69

Answer 1

Vapour density is calculated by dividing the molecular weight by 2. The molecular weight is the sum of atomic mass of each element present multiplied by the number of atoms of each element.

Complete step by step answer:
Ethyl alcohol or ethanol is an alcohol with the number of atoms 2. It is a volatile substance and is used as a good solvent and for synthesis of various organic compounds.
The molecular formula for the ethyl alcohol is ${\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}}$
The atomic mass of carbon is 12.
The atomic mass of hydrogen is 1.
The atomic mass of oxygen is 16.
The number of atoms of carbon is 2, the number of hydrogen is 6 and the number of atoms of oxygen is 1. Hence the molecular mass of ethyl alcohol will come out to be: $12 \times 2 + 16 \times 1 + 6 \times 1 = 46$
The vapour density will come out to be:
${\text{Vapour Density }} = \dfrac{{{\text{Molecular weight}}}}{2}$
$\dfrac{{46}}{2} = 23$

Hence, the correct option is B.
Note:
Vapour density is calculated in reference to the hydrogen gas molecule. We know hydrogen exists as diatomic gas and the molecular mass of hydrogen gas is 2. The vapour density is the ratio of the molecular weight of the substance to the molecular weight of hydrogen that is 2 and hence the formula derived which we used above.
The above formula is applicable for the pure substances. For example if we are only considering the ethyl alcohol then the formula is applicable. But in case if the mixture of gases is present then we cannot use the above formula for the calculation of the vapour density for that we need to consider the effective molecular weight.