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really difficult calculations- don't understand Mark scheme

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
Reply 1
Original post by tazmaniac97
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


In the question, there is 0.085 moles of carbon dioxide (or 2.74/44). The ratio of carbon to the carbon dioxide is 1:1 so you have the same number of moles of carbon as you do in carbon dioxide. Now that you know the moles of carbon you want to multiply it by the relative molecular mass of carbon (which is 12gmol^-1), to find the mass of carbon in the sample. So n=m/Mr so n*Mr = m. So you number of moles of carbon is 2.74/44 and you multiply this by 12.

Hope that makes sense! Edexcel Unit 5 synoptic right :L?
Reply 2
Original post by JoshL123
In the question, there is 0.085 moles of carbon dioxide (or 2.74/44). The ratio of carbon to the carbon dioxide is 1:1 so you have the same number of moles of carbon as you do in carbon dioxide. Now that you know the moles of carbon you want to multiply it by the relative molecular mass of carbon (which is 12gmol^-1), to find the mass of carbon in the sample. So n=m/Mr so n*Mr = m. So you number of moles of carbon is 2.74/44 and you multiply this by 12.

Hope that makes sense! Edexcel Unit 5 synoptic right :L?


Oh I see, because moles can also be applied to elements, since every carbon dioxide molecule has one carbon the number of molecules of carbon equals the number of molecules of carbon dioxide. Thanks :biggrin:

And no, it's unit 5 OCR :tongue:
(edited 10 years ago)

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