First we assume that the compound containing only C,H,and
O is combusted completely in the presence of excess oxygen, so that the only
things that can be produced are water and carbon dioxide.
From there we should back calculate the amount of
Hydrogen that is in the original sample by taking all of the hydrogen in the 0.239g
to came from the organic compound.
And since we know that the original mass of the sample
was .100g, we can also easily get a mass % H by taking the mass Hydrogen
calculated over the total original mass (.100 g)
So that:
0.239g H2O / (18.01 g/mol) = .01327 moles H20
.01327 Moles H20 * 2.02g H (per every mole H2O) = .0268g
H initially present in the sample
.0268g H / .100g sample = 26.8% H by mass
