Question

What is the number of moles of oxygen atom present in 126amu of $HN{{O}_{3}}$?

(A) 2

(B) $\dfrac{2}{{{N}_{A}}}$ 

(C) 6

(D) $\dfrac{6}{{{N}_{A}}}$ 

Answer 1

Hint: Number of moles of oxygen will be evaluated by the number of moles of nitric acid, using molecular mass of nitric acid and given weight of nitric acid.  The molecular weight of nitric acid is 63 g/mol.

Complete answer: 

The mole concept is a convenient method of expressing the amount of a substance. A mole of substance can be defined as the amount of a substance that contains $6.022\times {{10}^{23}}$ ions/molecules/atoms. This number is referred to as Avogadro Number or Avogadro Constant and is denoted as, ${{N}_{A}}$.

The number of moles in a substance can be represented as, 

\[moles=\dfrac{N}{{{N}_{A}}}\]

Where, N is the number of molecules/atoms/ions present in a substance.

The number of moles of a molecule may not always be equal to the number of moles of its constituents present in the substance. For example, One molecule of water has 2 atoms of hydrogen and one atom of oxygen, therefore one mole of water has 2 moles of hydrogen and one mole of oxygen.

Given is the 126amu of $HN{{O}_{3}}$, and now we have to tell the number of moles of oxygen present in it.

Molecular mass of $HN{{O}_{3}}$ is 63. So moles of $HN{{O}_{3}}$ given is,

\[mole=\dfrac{126}{63}=2\]

Therefore, we have 2 moles of $HN{{O}_{3}}$, since there are 3 atoms of oxygen present in one molecule of nitric acid, so moles of oxygen present in $HN{{O}_{3}}$ , will be $2 \times 3=6$, $6$ moles of oxygen present in $2$ moles of $HN{{O}_{3}}$.

And hence the number of moles =\[moles=\dfrac{N}{{{N}_{A}}}\]

And hence on substituting the values, we have

\[moles=\dfrac{6}{{{N}_{A}}}\]

So, the correct answer is “Option D”.

Note: To get the number of molecules of oxygen present in 2 moles of nitric acid, we will multiply the Avogadro number with the number of moles , that is Avogadro number multiplied with 6. Nitric acid is a strong acid as well as a strong oxidizing agent.