What is the number of moles of Oxygen atom in 126 amu of $HNO{}_3$

Calculating the number of moles of oxygen:

Step 1:

The molecular mass of $HN{O}_3=1+14+3\left(16\right)=63amu$

Given mass of $HN{O}_3=126amu$

Step 2:

$\text{Number of moles of}HN{O}_3=\frac{\text{Given mass}}{\text{Molecular mass}}$

$\text{Number of moles of}HN{O}_3=\frac{126}{63}=2moles$

Step 3:

Since 1 mole of $HN{O}_3$ contain $=3$ mole of oxygen

So, 2 mole of $HN{O}_3$ contain $=3\times 2=6$ mole of oxygen

Hence, 6 moles of oxygen are present in 126 amu of $HN{O}_3$