Why does oh have a subscript of 2 on the product side?

Which term refers to a property that depends only on the amount of a substance? ©@

If you burn 46.6 g of hydrogen and produce 416 g of water, how much oxygen reacted

Athe top of a hill, an athlete on a skateboard has x joules of mechanical energy. how much mechanical energy will she have at the bottom of the hill? ignore the effects of friction.