Potassium chlorate decomposes, on heating, to form potassium chloride and oxygen. When 24.5 g of potassium chlorate is decomposed completely, then 14.9 g of potassium chloride is formed. Calculate the mass of oxygen formed. Which law of chemical combination have you used in solving this problem?

Chemical reaction:

$\begin{array}{ccccc}2{\mathrm{KClO}}_3& \to & 2\mathrm{KCl}& +& 3{\mathrm{O}}_2\\ 24.5\mathrm{g}& & 14.9\mathrm{g}& & \end{array}$

So, mass of oxygen formed during the reaction:
Mass of reactants = 24.5 g
Mass of products = mass of the potassium chloride + mass of oxygen
24.5 = 14.9 + mass of oxygen
Mass of oxygen = 24.5 – 14.9 = 9.6 g
We have used the law of conservation of mass, proposed by Antoine Lavoisier, to solve this problem.