Hydrogen economy question! i've written my answer below but i'm missing half- equations and the advantages and disadvantages. i'm not even sure whether my answer is right at all. me with this. in advance!

Why does earth rotate? because earth is formed from cold gases collapsing due to gravity because the matter in the nebula that formed earth was spinning because earth forms more than 99% of the mass of the solar system because the hydrogen atoms inside the nebula fused to form helium

To find the enthalpy of a reaction in the lab, you measured the of the reactants and the change during the reaction.

The size (radius) of an oxygen molecule is about 2.0 ×10−10m. make a rough estimate of the pressure at which the finite volume of the molecules should cause noticeable deviations from ideal-gas behavior at ordinary temperatures (t= 300k ). assume that deviatons would be noticeable when volume of the gas per molecule equals the volume of the molecule itself.