the balanced equation;
2H₂O₂ ---> 2H₂O + O₂
the stoichiometry of H₂O₂ to O₂ is 2:1
this means that when 2 moles of H₂O₂ decompose, 1 mol of O₂ is formed.
Molar mass of H₂O₂ - 34 g/mol
the mass of H₂O₂ decomposed - 40.5 g
the number of moles of H₂O₂ decomposed - 40.5 g/ 34 g/mol = 1.19 mol
according to the stoichiometry of 2:1 ,
number of O₂ moles formed = 1.19 mol/2 = 0.60 mol