Hello, Lily,
I'll assume this is a decomposition reaction, KO2 = K + O2 , and that goes to completion. In this case, we'll obtain 1 mole of oxygen for every 1 mole of KO2. So let's determine how many moles are in 250 grams of KO2.
The molar mass of KO2 is 71.1 grams/mole.
Divide by the molar mass to find moles: 250grams/71.1grams/mole = 3.52 moles KO2
Multiply 3.52 moles by Avogadro's number to find molecules O2.
(3.52 moles O2)(6.02x1023molecules O2/mole) = 2.12x1024 molecules O2
Bob
