Nitric acid is composed of \text{HNO}_3 molecules. A sample weighing 4.50 g contains 4.30\times 10^{22} \text{HNO}_3 molecules. How many nitrogen atoms are in this sample? How many oxygen atoms are in 2.81 g of nitric acid?
Solution:
For the first part of the problem, we can see that in each \text{HNO}_3 molecule, there is one Nitrogen atom. Therefore, there are 4.30\times 10^{22} N atoms.
To figure out how many oxygen atoms are in 2.81 g of nitric acid, we will multiply the amount of nitric acid by the proportion which compares number of atoms to the amount of grams and multiply this by the proportion of Oxygen atoms to 1 \text{HNO}_3 molecule. Yes, we know this is hard to follow, however, we hope the following will make it clear. We will write it out:
2.81g\times \frac{4.30\times 10^{22}\,\text{HNO}_3\,\text{molecules}}{4.50g\,\text{HNO}_3}\times \frac{3\,\text{O}\,\text{atoms}}{1\,\text{HNO}_3\,\text{molecule}}
=8.05\times 10^{22} of oxygen atoms.
This question can be found in General Chemistry, 9th edition, chapter 2, question 2.66
