4
$\begingroup$

I'm reading Shaw's "Trace Elements in Magmas" and in his page 16 he writes the following:

...It is necessary to look at the relationship between $c_i$ and $x_i$, i.e. the weight and molar concentrations. In any phase of weight $W$ the weight of component $i$ is $w_i$ and if the formula weight of $i$ is $M_i$, then the number of gfu (gram-formula unit concentration) of $i$ is $n_i$, where $n_i = \frac{w_i}{M_i}$...

What does he mean when he mentions the "formula weight" ($M_i$) and the gfu (gram formula unit)?

Improve this question
$\endgroup$

1 Answer 1

Reset to default
6
$\begingroup$

$M_i$ is the molar mass of the ith element. Dividing $w_i$ by $M_i$ yields the number of moles of that element in the sample.

There are two primary ways of looking at abundances. One is by weight (or mass), the other is by number of atoms. Consider beryllium and uranium. Their abundances are very similar when looked at in terms of mass: 1.9 parts per million (ppm) for beryllium versus 1.8 ppm for uranium. In terms of molar abundance, they aren't so similar. In this sense, beryllium's abundance is 4.3 ppm; for uranium, it's only 0.15 ppm.

Improve this answer
$\endgroup$
4
  • $\begingroup$ Obviously. Beats me as to why he used such obscure terminology and not the usual moles and molar weight/mass. $\endgroup$
    – Gimelist
    Commented Nov 7, 2014 at 11:27
  • 2
    $\begingroup$ I've never run across the term "gram formula unit" before, but apparently that term is a long-winded synonym for "mole", particularly in geochemistry. $\endgroup$
    – David Hammen
    Commented Nov 7, 2014 at 11:33
  • $\begingroup$ Apparently where? I'm a geochemist (albeit a young one) and I've yet to encounter it. $\endgroup$
    – Gimelist
    Commented Nov 7, 2014 at 17:03
  • 1
    $\begingroup$ "Mole" as a unit is fairly new. It was added to the International System of Units (aka metric system) in 1971. Before that scientists used things such as atomic weight (which is still used), gram-molecules, gram-atoms, and gram-formula. The article Milton, "A new definition for the mole based on the Avogadro constant: a journey from physics to chemistry." Philosophical Transactions of the Royal Society A 369.1953:3993-4003 (2011) discusses the past, present, and future of assessing quantity of matter. $\endgroup$
    – David Hammen
    Commented Nov 7, 2014 at 17:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.