Question

What is the mass of one molecule of oxygen?
A. $5.31 \times \mathop {10}\nolimits^{ - 23} $ g
B. $5.33 \times \mathop {10}\nolimits^{23} $ g
C. $5.25 \times \mathop {10}\nolimits^{23} $ g
D. $5.02 \times \mathop {10}\nolimits^{ - 23} $ g

Answer 1

Hint: The chemical formula of oxygen is $\mathop O\nolimits_2$ and one molecule of oxygen contains two oxygen atoms. Also, the molecular mass of an oxygen atom is 16 amu.

Complete step by step solution:
It is known that oxygen belongs to group 16 of periodic table.
The atomic number of an oxygen atom is 8. It has 8 electrons, 8 protons and 8 neutrons.
We also know that the atomic mass of an oxygen atom is 16 amu where amu stands for atomic mass unit.
Atomic mass unit is defined as $\dfrac{1}{{12}}$ of the mass of an atom of carbon-12(C-12)”.The carbon_12 or C-12 has 6 protons and 6 neutrons in its nucleus.
In precise terms we can say that one amu is the average of the proton rest mass and the neutron rest mass which is equal to $1.6605 \times \mathop {10}\nolimits^{ - 24} $ g.
As we already know, the mass of one oxygen atom is 16 amu and 1 amu=$1.6605 \times \mathop {10}\nolimits^{ - 24}$ g.
So, mass of one oxygen atom
$
   = 16 \times 1.6605 \times \mathop {10}\nolimits^{ - 24} \\
   = 2.6568 \times \mathop {10}\nolimits^{ - 23} g \\
$
Also, one molecule of oxygen contains two oxygen atoms.
Therefore, mass of one molecule of oxygen =$2 \times mass{\text{ }}of{\text{ }}one{\text{ }}oxygen{\text{ }}atom$
$
   = 2 \times 2.6568 \times \mathop {10}\nolimits^{ - 23} \\
   = 5.31 \times \mathop {10}\nolimits^{ - 23} g \\
$

Hence, from above it is clear that option A is the correct answer.

Note: It should be noted that before 1961, isotope of O-16 was taken as reference instead of C-12 in order to calculate the value of 1 amu.
Also, it should be remembered that atomic mass is slightly less than actual mass of atom because in calculation of atomic mass the mass of electrons is not included because it is negligible when compared to proton and neutron.