Calculate the percentage composition of each element in Potassium chlorate, KClO₃.

Relative molecular mass of KClO₃
   = 39 + 35.5 + 3 x 16
   = 39 + 35.5 + 48
   = 122.5 amu

122.5 g of KClO₃ contains 39 g of Potassium

∴ 100 g of KClO₃ contains $\dfrac{39 \times 100}{122.5}$ g of Potassium

= $\dfrac{39000}{1225}$ = 31.83 g of Potassium

122.5 g of KClO₃ contains 35.5 g of Chlorine

∴ 100 g of KClO₃ contains $\dfrac{35.5 \times 100}{122.5}$ g of Chlorine

= $\dfrac{35500}{1225}$ = 28.98 g of Chlorine

122.5 g of KClO₃ contains 48 g of Oxygen

∴ 100 g of KClO₃ contains $\dfrac{48 \times 100}{122.5}$ g of Oxygen

= $\dfrac{48000}{1225}$ = 39.18 g of Oxygen

∴ In KClO₃ : K = 31.83%, Cl = 28.98% and O = 39.18%