What is the percentage by mass of nitrogen in sodium nitrate?

The formula for sodium nitrate is NaNO3, showing that each formula unit contains one sodium atom, one nitrogen atom, and three oxygen atoms. The gram atomic masses of sodium, nitrogen, and oxygen are 22.9898, 14.0067, and 15.9994 respectively. Therefore, the percentage by mass of nitrogen in the compound is:

100{14.0067/[14.0067 + 22.9898 + 3(15.9994)]} or about 16.4795, to the justified number of significant digits.