Question

The mass of one oxygen molecule is:
A. $2.656\times {{10}^{-23}}gm$
B. $5.312\times {{10}^{-23}}gm$
C. $1.66\times {{10}^{24}}gm$
D. $32\,gm$

Answer 1

Oxygen (${{O}_{2}}$) is a diatomic molecule and mass of one oxygen atom equals 16 amu. We multiply 16 by 2 to find the mass of two oxygen atoms. Then we divide the mass by the Avogadro number.
- In general, Avogadro Number = $6.023\times {{10}^{23}}\,mo{{l}^{-1}}$.

Complete Solution :
Mass of one oxygen atom equals 16 amu.
Therefore, mass of a ${{O}_{2}}$ molecule = 2*16 = 32 amu
$\Rightarrow \,Mass\,of\,one\,\,oxygen\,molecule\,=\,\dfrac{32}{Avogadro\,number}$
$\Rightarrow \,Mass\,of\,one\,\,oxygen\,molecule\,=\,\dfrac{32}{6.023\times {{10}^{23}}}$
$\Rightarrow \,Mass\,of\,one\,\,oxygen\,molecule\,=\,5.312\times {{10}^{-23}}\,gm$
So, the correct answer is “Option B”.

Note: (i) The number of units in one mole of any substance is equal to Avogadro’s number that is $6.023\times {{10}^{23}}$. The units may be ions, molecules, electrons or atoms or anything depending upon the nature of the substance.
(ii) The SI unit of Avogadro’s Number is the reciprocal mol that is mol-1. It gets its name from the Italian scientist Amedeo Avogadro. According to Avogadro's law, the number of atoms (or molecules or ions or electrons) in a substance is proportional to its physical mass.
(iii) Let’s take an example to understand this concept in a better way:
When one oxygen atom combines with two atoms of hydrogen to create one molecule of water, one mole of oxygen ($6.023\times {{10}^{23}}$ of O atoms) will combine with two moles of hydrogen ($2\times 6.023\times {{10}^{23}}$of $H$ atoms) to make one mole of water (${{H}_{2}}O$).
(iv) The mass of one mole of a substance is equal to that substance’s molecular weight.
(v) The atomic mass unit (amu) is defined as 1/12 of the mass of the atom of carbon-12 which is the most common isotope of carbon. Carbon-12 has 6 photons, 6 electrons and 6 neutrons.
(vi) FUN FACT: Mole Day is celebrated every year by chemists throughout the world. Since Avogadro's number is $6.023\times {{10}^{23}}$, it only makes sense that the holiday starts at 6:02 am every Oct 23.