Answer: the mass percentage of C in codeine is 72.21%.
Explanation: Molar mass of C = 12.011 g/mol
Molar mass of CisHziNO = 299.368 g/mol (whole of molar masses of all molecules within the compound)
Number of C molecules in codeine = 18
Mass of C in codeine = 12.011 g/mol x 18 = 216.198 g/mol
Mass rate of C in codeine = (216.198 g/mol ÷ 299.368 g/mol) x 100% = 72.21%