It has been estimated that 93% of all atoms in the entire universe are hydrogen and that the vast majority of those remaining are helium. Based on only these two elements, estimate the mass percentage composition of the universe.

Explanation

Given that out of 100 atoms of H and He, 93 atoms are of H and 7 atoms are of He, that is, the number of moles of H and He atoms, out of 100 moles, are 93 and 7 respectively.
Mass of H `=93 xx 1 = 93 g`
Mass of He `=7 xx 4 = 28 g`
`therefore` mass percentage of H `=93/(93 + 28) xx 100 = 76.86%`
`therefore` mass percentage of He `=23.14%`