Question

Calculate the mass percentage composition of the elements in nitric acid. (H = 1, N = 14, O = 16).

Answer 1

Hint: The percentage of the elements or constituents 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.

Complete answer:
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.
$\text{Percentage of the element or constituents = }{\displaystyle \frac{\text{no}\text{. of parts by mass of the elements or constituents}}{\text{Mol}\text{. mass of the compound}}}\text{ x 100}$
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 = ${\displaystyle \frac{\text{mass of hydrogen}}{\text{molar mass of nitric acid}}}\text{ x 100}$
Mass percentage of hydrogen = ${\displaystyle \frac{1}{63}}\text{ x 100 = 1}\text{.586 }$
Mass percentage of nitrogen = ${\displaystyle \frac{\text{mass of nitrogen}}{\text{molar mass of nitric acid}}}\text{ x 100}$
Mass percentage of nitrogen = ${\displaystyle \frac{14}{63}}\text{ x 100 = 22}\text{.22 }$
Mass percentage of oxygen = ${\displaystyle \frac{\text{mass of oxygen}}{\text{molar mass of nitric acid}}}\text{ x 100}$
Mass percentage of oxygen = ${\displaystyle \frac{48}{63}}\text{ x 100 = 76}\text{.19 }$
So, the mass percentage of hydrogen is 1.58%, nitrogen is 22.22%, and oxygen is 76.19%.

Note: In some of the compounds the mass composition of a molecule is calculated like $H_{2}O$ etc. In some compounds the mass composition of ions is calculated, the mass of ions is the same as that of its neutral atom or molecule.