Eugenol is used as a painkiller in dentistry. It is an organic compound of C, H
and O.A sample of 1.394 g of eugenol was analysed by burning in oxygen to form 3.74 g of CO2 and 0.918 g of H2O. The relative molecular mass of eugenol was shown to be 164 using a mass spectrometer. Calculate the molecular formula of eugenol.
Mark scheme:
mass C = 12 × 3.74/44 = 1.02 g (<-- what exactly are they doing her!? What formula are they using (if any)?
moles CO2 = 3.74/44 = 0.085 mol
mass H = 2/18 × 0.918 = 0.102 g
moles H2O = 0.918/18 = 0.051 mol
ratio C : H = 1.02/12 : 0.102/1 = 0.0850 : 0.102 = 5 : 6 / 10 : 12
ratio CO2 : H2O = 5 : 3 / 10 : 6
mass O = 1.394 – (1.020 + 0.102) = 0.272 g
using 1.394 g eugenol and Mr = 164, shows that 1 molecule contains 2 atoms of O. Molecular formula = C10H12O2