Chlorous acid, ##HClO_2##, contains what percent hydrogen by mass?

##1.472%##
The first thing to do here is make a note of all the atoms that make up a molecule of chlorous acid, ##”HClO”_2##.
As the molecular formula suggests, you have
one atom of hydrogen
one atom of chlorine
two atoms of oxygen
Now, if one molecule contains this many atoms, it follows that one mole of chlorous acid will contain
one mole of hydrogen atoms
one mole of chlorine atoms
two moles of oxygen atoms
This means that you can use the molar mass of chlorous acid and the molar mass of hydrogen to help you determine how many grams of hydrogen you’d get per mole of chlorous acid.
Chlorous acid has a molar mass of ##”68.4597 g/mol”## and hydrogen has a molar mass of ##”1.00794 g/mol”##. This means that every mole of chlorous acid ahs a mass of ##”68.4597 g”##, out of which ##”1.00794 g”## come from hydrogen.
The of hydrogen in chlorous acid will thus be
##(1.00794color(red)(cancel(color(black)(“g”))))/(68.4597color(red)(cancel(color(black)(“g”)))) xx 100 = color(green)(1.472%)##