Chlorous acid, $ HCl{O_2} $ , contains what percent hydrogen by mass?

Hint: The molar mass of hydrogen is $ 1gmo{l^{ - 1}} $ and the molar mass of chlorous acid is $ 68.5gmo{l^{ - 1}} $ . Dividing the molar mass of hydrogen with the molar mass of chlorous acid and then multiplied by the factor $ 100 $ gives the percent of hydrogen by mass.

Complete Step By Step Answer:
Chlorous acid is a weak acid with the molecular formula of $ HCl{O_2} $ , in which there is one hydrogen atom, one chlorine atom, and two oxygen atoms.
Hydrogen is an element with atomic number $ 1 $ and has the molar mass of $ 1gmo{l^{ - 1}} $
Chlorine is an element with atomic number $ 17 $ and has the molar mass of $ 35.5gmo{l^{ - 1}} $
Oxygen is an element with atomic number $ 8 $ and has the molar mass of $ 16gmo{l^{ - 1}} $
As chlorous acid consists of two oxygen atoms. Thus, the molar mass of oxygen atoms in chlorous acid will be $ 2 \times 16gmo{l^{ - 1}} = 32gmo{l^{ - 1}} $
The molar mass of chlorous acid is given by the sum of the molar mass of hydrogen, chlorine, and two oxygen atoms.
Molar mass of chlorous acid is $ 1 + 35.5 + 32 = 68.5gmo{l^{ - 1}} $
The percent of hydrogen in chlorous acid will be obtained by dividing the molar mass of hydrogen by the molar mass of chlorous acid.
 $ \% H = \dfrac{{{M_H}}}{{{M_{HCl{O_2}}}}} \times 100 $
By substituting the molar masses of hydrogen, chlorous acid in the above formula,
 $ \% H = \dfrac{1}{{68.5}} \times 100 = 1.472\% $
Thus, the Chlorous acid, $ HCl{O_2} $ , contains $ 1.472 $ percent hydrogen by mass.

Note:
While calculating the percentages of any particular atom in any compound, the molar mass should be taken by considering the number of that particular atom in the compound. As we are calculating the percent of hydrogen by mass, only one atom of hydrogen was considered. As chlorous acid consists of one hydrogen atom.