I am reviewing moles, and came across this sentence in my book:

One mole of a compound has a mass in grams equal the molecular weight of the compound expressed in amu and contains $6.022\times10^{23}$ molecules of that compound.

Now, I perfectly understand the second part of the sentence. 1 mole of compound has $6.022\times10^{23}$ molecules. But how do $6.022\times10^{23}$ molecules have a mass same as the molecular weight of compound?
Ex. Molecular weight of $\ce{SOCl2}$ is $119\ \mathrm{amu}$. That is the mass of 4 atoms. How do $6.022\times10^{23}$ molecules have the same mass?

  • 4
    $\begingroup$ The wording in the sentence is poor. For the record... One molecule of $\ce{SOCL2}$ has a mass of 119 amu, but one mole of $\ce{SOCL2}$ has a mass of 119 grams and has $6.022*10^{23}$ molecules. $\endgroup$
    – MaxW
    Aug 30, 2016 at 22:37
  • $\begingroup$ In simpler terms, if you have one mole of something, then the molecular weight of that substance will be the same as the sample weight. If you weight one mole of carbonic acid, it would be 62g which would be the same as its molecular weight (in amu). The correct term should be "molecular mass". I think that was also misleading. $\endgroup$
    – Singh
    Aug 31, 2016 at 5:47

1 Answer 1


Well you can actually calculate it like this $$1\ \mathrm{amu}=1.66\times10^{-27}\ \mathrm{kg}$$ Therefore $$119\ \mathrm{amu}=119\times1.66\times10^{-27}\ \mathrm{kg}$$ But for 1 mole, multiplying by Avogadro number you get mass of one mole as $$\begin{align} 119\times1.66\times10^{-27}\times6.022\times10^{23}&=0.11899\ \mathrm{kg}\\ &=118.99\ \mathrm g\\ &\approx119\ \mathrm g \end{align}$$

  • $\begingroup$ Welcome to Chemistry! Please visit this page, this page and this ‎one on how to format your posts better with MathJax and Markdown, then edit this answer to include proper formatting. Thanks $\endgroup$
    – M.A.R.
    Aug 31, 2016 at 14:52

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