Question

If a compound contains two oxygen atoms, four carbon atoms and number of hydrogen atoms is double of carbon atoms, the vapour density is:
(A) 88
(B) 44
(C) 132
(D) 72

Answer 1

Since the question is in word form, first we will derive out the compound from the number of individual atoms. Then we will calculate the molecular mass of the compound.
Then, ${\rm{vapour density = }}\dfrac{{{\rm{Molecular mass of the compound}}}}{{\rm{2}}}$

Complete step by step answer:
-Let us understand the term of vapour density first.
-The density of a particular gas or vapour relative to that of hydrogen at the same pressure and temperature is called vapour density.
-Now, what does this mean?
-The relative weight of a vapour compared to the air has an arbitrary value of one. When the vapor density of gas is less than one, it is said to rise in air. When the vapor density of gas is greater than one, it is said to sink in air.
-The relation in between the molecular mass of compound and vapour pressure is as follows:
${\rm{vapour density = }}\dfrac{{{\rm{Molecular mass of the compound}}}}{{\rm{2}}}$
Now, let us find out the compound with the information provided to us:
-there are 2 oxygen atoms, four carbon atoms and eight hydrogen atoms (as it is twice than the number of carbon atoms).
Consider, ${C_4}{H_8}{O_2}$
Now, let us calculate the molecular mass
$Molecular{\rm{ }}mass = 4{\rm{ }} \times {\rm{ }}12{\rm{ }} + {\rm{ }}8{\rm{ }} \times 1{\rm{ }} + {\rm{ }}2{\rm{ }} \times {\rm{ }}16$
$Molecular{\rm{ }}mass{\rm{ }} = 88$
Thus, $vapour{\rm{ density}} = \dfrac{{88}}{2} = 44$

Therefore, the correct answer is option (B) 44.

Note: Do not confuse density with vapour density. Density is usually used for a solid or liquid. This is calculated by dividing the mass of the substance by the volume. Vapour density refers to a gas or vapour. It's a measure of the mass compared with an equal volume of hydrogen.