Start with the balanced chemical equation for the of potassium chlorate,
The important thing to notice here is that
Since you know the volume and conditions under which the oxygen gas was produced, you can use the equation to determine how many moles were produced.
At NTP, or Normal Temperature and Pressure, the pressure is set at 1 atm and the temperature at
Use oxygen's molar mass to determine how many grams were produced
SIDE NOTE Do not forget to convert the temperature to Kelvin and the volume to liters!
The mass of potassium chlorate that underwent decomposition can be found in a similar manner. First, determine the number of moles by using ratio that exists between potassium chlorate and oxygen gas
The mass of the compound before decomposition was
The number of moles of potassium chlorate produced by the reaction will be equal to the number of moles of potassium chlorate
The mass of potassium chloride is
All three values are rounded to three , the number of sig figs you gave for the volume of oxygen gas produced.