Activation energy is the minimum amount of energy needed to start a chemical reaction
A Catalyst is a material that increases the rate of a reaction by lowering the activation energy. In other words, a catalyst act to provide an alternative route for the reaction which has a lower activation energy.
Since moles are just "some number of particles", and since one O2 molecule contains enough oxygen for two H2O molecules, you need 2.5 moles of O2 for 5 moles of H2O.
When two of the same element combine it will always be a covalent bond between them and since sulfur has two lone electrons it will make a double bond between the two to have a full octect
