Question 12 (b) is a challenging question at this stage. You should consider how many moles of hydrogen peroxide will produce 1 mole of oxygen. From the molar volume of oxygen (this figure tells you what volume 1 mole of oxygen occupies), you will then be able to calaculate how many moles of oxygen have been produced in the experiment. From those two pieces of information and the familiar equations relating moles and mass, you should be able to determine the mass of hydrogen peroxide originally present.
