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The percentage of any element or constituent in a compound is the number of parts by mass of that element or constituents present in 100 parts by mass of the compound. It can be calculated by the two steps:

Step 1: Calculated the molecular mass of the compound from its formula by adding the atomic masses of the elements present.

Step 2: Calculate the percentage of the elements or the constituents by applying the following relation:

It can be calculated by dividing the number of parts of the mass of the element or constituents with the molar mass of the compound multiplied to 100.

$\text{Percentage of the element or constituents = }\dfrac{\text{no}\text{. of parts by mass of the elements or constituents}}{\text{Mol}\text{. mass of the compound}}\text{ x 100}$

The formula of nitric acid is $HN{{O}_{3}}$

The molar mass of hydrogen is 1, the molar mass of nitrogen is 14, and the molar mass of oxygen is 16.

The molar mass of nitric acid = 1 + 14 + 3(16) = 1 + 14 + 48 = 63.

Mass percentage of hydrogen = $\dfrac{\text{mass of hydrogen}}{\text{molar mass of nitric acid}}\text{ x 100}$

Mass percentage of hydrogen = $\dfrac{1}{63}\text{ x 100 = 1}\text{.586 }\!\!%\!\!\text{ }$

Mass percentage of nitrogen = $\dfrac{\text{mass of nitrogen}}{\text{molar mass of nitric acid}}\text{ x 100}$

Mass percentage of nitrogen = $\dfrac{14}{63}\text{ x 100 = 22}\text{.22 }\!\!%\!\!\text{ }$

Mass percentage of oxygen = $\dfrac{\text{mass of oxygen}}{\text{molar mass of nitric acid}}\text{ x 100}$

Mass percentage of oxygen = $\dfrac{48}{63}\text{ x 100 = 76}\text{.19 }\!\!%\!\!\text{ }$

So, the mass percentage of hydrogen is 1.58%, nitrogen is 22.22%, and oxygen is 76.19%.