51.554%
First, look up the atomic weights of all the involved elements
atomic weight carbon = 12.0107
atomic weight hydrogen = 1.00794
atomic weight oxygen = 15.999
Now calculate the molar mass of ethylene glycol
m = 2 * 12.0107 + 6 * 1.00794 + 2 * 15.999 = 62.06704 g/mol
Since there's 2 oxygen atoms in a single molecule of c2h6o2, calculate the mass of those 2 oxygen atoms.
m = 2 * 15.999 = 31.998 g/mol
Finally divide the mass of the oxygen by the molar mass of c2h6o2.
31.998 / 62.06704 = 0.515539327 = 51.5539327%
Since the lowest number of significant digits I have is for oxygen at 5, round the result to 5 significant figures, giving 51.554%
