The percent of potassium in the compound KClO₃ = 32%
Proust states the Comparative Law that compounds are formed from elements with the same Mass Comparison, so that compounds have a fixed composition of elements
[tex]\tt \%mass=\dfrac{Ar~element\times number~of~element~in~compound}{MW~compound}\times 100\%[/tex]
We determine the molecular mass of KClO₃ then we determine the Ar from K (potassium) and the amount in the compound
MW KClO₃ : 122,55 g/mol
The amount of K in KClO₃ = 1
So %K :
[tex]\tt =\dfrac{39}{122,55}\times 100\%=31.8\%\approx 32\%[/tex]