In order to compare the mass fractions of oxygen in the compounds given to us, we will find the mass fraction of oxygen in each of the substances. The general formula for finding the mass fraction of an element in a substance is:
w = ((n * Ar): M) * 100%; where w is the mass fraction of an element in a substance (measured as a percentage), n is the number of atoms of an element in a substance, Ar is the relative atomic mass of an element, M is the relative molecular mass of a substance.
w (O in CO) = ((1 * 16g / mol): 28g / mol) * 100% = (16: 28) * 100% = 57.1%;
w (O in H2O) = ((1 * 16g / mol): 18g / mol) * 100% = (16: 18) * 100% = 88.8%;
w (O in CO2) = ((2 * 16g / mol): 44g / mol) * 100% = (32: 44) * 100% = 72.7%;
Mass fraction of oxygen is equal to 88% (rounded up) in H2O.
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