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%