Use Ideal gas law to solve. PV=mRT you have pressure (1.02atm), temperature (295K or 22°C), mass of KClO3(11.83g) , and universal gas constant on a molar mass basis (R=R_bar/M). Just make sure you units are consistent and use the ideal gas equation to solve for Volume. M, should be the total Molar mass for KClO3.
