Here is a KClO3 worked example. Just follow the steps.
http://www.jiskha.com/science/chemistry/stoichiometry.html
2KClO3 ----> 2KCl + 3O2
Calculate the number of grams of KClO3 decomposed given an initial sample mass of 2.16g and a residue mass of 1.71g?
2 answers
5.48 litres