First, convert the amount of KNO3 to moles using the molar mass: 58.6 g / 101.11 g/mol = 0.5796 mol KNO3
Next, using the given balanced reaction, use the stoichoimetric ratio to calculate the number of mole of oxygen that will be produced. From the reaction, for every 4 moles of KNO3 that decomposes, there are 5 moles of O2 formed. So, 0.5796 mol KNO3 x (5/4) = 0.7254 moles O2
Therefore, when 58.6 g of KNO3 decomposes, there are 0.7254 moles of O2 that will be formed.