What is the total number of moles of o2 produced from the complete decomposition of 8 moles h2o2

We will solve this question using stoichiometry we will first set up the equation of decomposition which is 2 H2O2 ==> 2 H20 + O2 using this equation we see that 2 mole of H2O2 will produce 1 mole of O2 thus 8 mole of H2O2 will produce 4 mol of O2 so the answer is 4 mol

The mass has escaped into a gas which you can not see so the law stays true.