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Could I get an explanation of Avogadro's number and how it relates to determining the mass of a substance? My chemistry textbook only serves to confuse me and the Wikipedia article is aimed towards a more wordy audience. I understand that Avogadro's number is $6.022 \times 10^{23}$, but I'm not sure how to relate that to determining the amount of atoms in a particular substance. Is it a direct proportion between the weight of C-12 and the weight of whatever substance you're attempting to determine the amount of atoms in?

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Hi Elliot, and welcome to Physics Stack Exchange! In reference to the original version of your question: just because there's no chemistry SE site doesn't make chemistry questions acceptable here. But you got "lucky," in a sense, because this is really a question about units, which we do consider on topic here. –  David Z Nov 1 '11 at 18:06
Okay, thank you! Where, exactly, would I ask a chemistry-related question in the future? –  Elliot Bonneville Nov 1 '11 at 18:10
There is a chemistry proposal on Area 51. Once that launches, you would be able to ask chemistry questions there. But until then, I don't believe there is any place on the Stack Exchange network for them. –  David Z Nov 1 '11 at 18:12
Okay, thanks. I'll reserve my chemistry questions for that site, then. –  Elliot Bonneville Nov 1 '11 at 18:15
And don't forget to commit to the proposal to help it along! –  David Z Nov 1 '11 at 18:20
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1 Answer

up vote 3 down vote accepted

The simplest way to think of Avogadro's number is as a unit conversion factor. Just as there are 2.54 centimeters in an inch, there are Avogadro's number atomic mass units in a gram.

Accordingly, if you know the atomic mass of an element you can count the number of atoms in one gram of it. Let's take carbon as an example. The most common isotope of carbon has an atomic mass of 12. There are rare forms of carbon that are heavier because they have extra neutrons, but we'll ignore them here. Thus 1 gram of carbon contains $\frac{\text{Avogadro's Number}}{12}$ carbon atoms. Likewise if we had $6.02\times10^{23}$ carbon atoms, we'd have 12 grams of carbon.

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Aha. So if I wanted to find the amount of atoms in a gram of, say, oxygen, I would put something like $\frac{(6.022\times10^{23})}{16}$ oxygen atoms? –  Elliot Bonneville Nov 1 '11 at 17:35
@ElliotBonneville: Yup, you've got it. I would, however, highly recommend always always always using the units. $6.022 \times 10^{23} \left[\frac{\mbox{atoms}}{\mbox{mol}}\right] \cdot \frac{1}{16} \left[\frac{\mbox{mol}}{\mbox{g}}\right]$. –  AdamRedwine Nov 1 '11 at 19:24
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