Question

Which of the following gases is lighter than air?
A. Oxygen
B. Hydrogen chloride
C. Ammonia
D. Chlorine

Answer 1

Hint: Here, we will first of all, we will see the definition of atomic mass. Then calculate the atomic mass of given compounds . Assuming that air consists mainly of nitrogen(78%). The Gases which have less atomic mass than that of air will be lighter than the air.

Complete answer:
First of all, we will see the definition of atomic mass.
Atomic mass, the quantity of matter contained in an atom of an element. It is expressed as a multiple of one-twelfth the mass of the carbon-12 atom, $1.992646547 \times {10^{−23}}$ gram, which is assigned an atomic mass of 12 units. In this scale 1 atomic mass unit (amu) corresponds to $1.660539040 \times {10^{−24}}$ gram.
For example, the atomic mass of carbon(C) is 12 units.
Let us consider us consider that air mainly consists of nitrogen gas (${N_2}$)
Atomic mass of nitrogen gas (${N_2}$) = $2 \times 14$ =28
Atomic mass of hydrogen chloride(HCl) = 1+35.5=36.5
Atomic mass of Ammonia ($N{H_3}$ ) = 14+3$ \times $1=17
Atomic mass of Chlorine gas (Cl) = 35.5
Therefore, we can say that only Ammonia out of four given options has less mass than that of air.

So. Option C is the correct one.

Note: You should remember the atomic mass of some important elements.

On going down the group, the atomic mass increases due to an increase in the number of protons and neutrons.