Question

Which of the statements are true?
A. Law of constant composition is true for all types of compounds.
B. Molar volume of a gas at standard conditions is $22.4{\text{ L}}$.
C. Vapour density of a gas is twice its molecular mass.
D. Atomic masses of most elements are fractional.

Answer 1

-The law of constant composition states that all the samples for a given chemical compound are composed of the same elements combined in the same proportions.
-When the pressure is $1{\text{ atm}}$ and the temperature is $273{\text{ K}}$the molar volume of any gas is $22.4{\text{ L}}$.
-The vapour density of a gas is given by the formula,
${\text{Vapour density}} = \dfrac{{n \times {\text{Molecular mass of molecules of gas }}}}{{n \times {\text{Molecular mass of molecules of hydrogen gas }}}}$
-Many of the elements that occur are a mixture of isotopes.

Complete step by step answer:
Step 1:
The law of constant composition states that all the samples for a given chemical compound are composed of the same elements combined in the same proportions.
The law of constant composition is applicable to a large number of compounds. But the law is not applicable for:
Non-stoichiometric compounds.
Compounds made up of isotopes of different elements.
Thus, the statement ‘law of constant composition is true for all types of compounds’ is not true.
Step 2:
When the pressure is $1{\text{ atm}}$ and the temperature is $273{\text{ K}}$the molar volume of any gas is $22.4{\text{ L}}$.
Thus, the statement ‘molar volume of a gas at standard conditions is $22.4{\text{ L}}$’ is true.
Step 3:
The density of any vapour relative to the vapour density of the hydrogen gas at same temperature and pressure is known as the vapour density of the gas.
The vapour density of a gas is given by the formula,
${\text{Vapour density}} = \dfrac{{n \times {\text{Molecular mass of molecules of gas }}}}{{n \times {\text{Molecular mass of molecules of hydrogen gas }}}}$
Where $n$ is the number of molecules.
Thus, the vapour density of one molecule of any gas is,
${\text{Vapour density}} = \dfrac{{{\text{Molecular mass of molecules of gas }}}}{2}$
Thus, vapour density of a gas is half of its molecular mass.
Thus, the statement ‘vapour density of a gas is twice of its molecular mass’ is not true.
Step 4:
Many of the elements that occur are a mixture of isotopes. These isotopes have different atomic masses. Thus, the atomic masses of these elements are fractional.
Thus, the statement ‘atomic masses of most elements are fractional’ is true.
Thus, the correct options are option (B) and option (D).

Note: The sum of the products of masses of all the isotopes and their natural abundances is the average atomic mass.