I am interested in learning a little about mass defect and calculating the molar mass of ethyl alcohol.

I found this table of atomic weights and would like to use it to compute the mass of the average ethyl alcohol molecule. How wrong is it to simply say

$$m_{\text{C}_2\text{H}_5 \text{O}\text{H}}=2m_{\text{C}}+6m_{\text{H}}+m_{\text{O}}. $$

How does binding energy affect the mass of one molecule? Can I argue like this: We start with one molecule of ethyl alcohol with mass $m_{\text{molecule}}$. Now, we "tear apart" a hydrogen atom from it. Let the Energy required to do that be $E_1.$ We keep doing this until we have 9 isolated atoms. Let's say the total energy required to do that was $\Delta E = \sum_i E_i$. Can I now say

$$ m_{\text{molecule}}=m_{\text{isolated}}+\frac{\Delta E}{c^2} ? $$

  • 1
    $\begingroup$ In general, molecular binding energies are at the order of 0.1--10 eV, while the mass ot the nuclei/atoms is in the order of 1--10 GeV/$c^2$ So we're talking about mass deficiencies about 10 orders of magnitude smaller than the mass of the molecule. $\endgroup$ – Andréas Sundström Oct 11 at 9:07
  • $\begingroup$ @AndréasSundström Do you have a source for me? $\endgroup$ – ty. Oct 11 at 11:32
  • $\begingroup$ Your plus sign should be a minus: molecule is slightly lighter, not heavier, than sum if weights of atoms. $\endgroup$ – Ben51 Oct 11 at 13:31
  • $\begingroup$ Do you have a source for me? Look up 'Heat of Formation' (of chemical compounds) $\endgroup$ – Gert Oct 11 at 13:46
  • $\begingroup$ @ty. Well I'm no chemist, but as Gert said, you can have a look at the enthalpy of formation for the energy released when forming a molecule from free atoms. Then the mass of the nuclei/atoms is some approximate integer multiple of the proton mass, which is 0.938 GeV/$c^2$. $\endgroup$ – Andréas Sundström Oct 11 at 14:37

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